An experimental stem cell therapy derived from human placentas will begin early testing in patients with the coronavirus, a New Jersey biotech company said Thursday.

The treatment, being developed by the company Celularity, has not yet been used on any patients with symptoms of Covid-19, but it has caught the attention of Rudy Giuliani, President Trump’s personal lawyer. Mr. Giuliani recently featured an interview with the company founder on his website and said on Twitter that the product has “real potential,” while also criticizing the Food and Drug Administration for not moving more quickly to approve potential remedies.

There is no proven treatment for the respiratory disease, but several experimental approaches, including old malaria drugs and H.I.V. antivirals, are being tested in patients around the world.

Celularity has also enthusiastically publicized the news of its early-stage trial for its treatment, known as Cynk-001. In an email Wednesday to a reporter, its public relations firm described a development as the “first F.D.A. approval for Covid-19 cell therapy.” The agency’s decision, however, merely gives a green light for its product to be used in a clinical trial, not widely prescribed to patients.

In recent weeks, the established scientific process of evaluating a drug’s safety and effectiveness has been upended by Mr. Trump, who has repeatedly promoted the potential of two long-used malaria drugs that have shown mainly anecdotal evidence of helping patients. On Saturday, the F.D.A. took the unusual step of approving those drugs to treat hospitalized patients with coronavirus on an emergency basis, even though no significant clinical trials have yet been done.

The early trial by Celularity — which will primarily evaluate safety, as well as an initial look at efficacy — will test its therapy in up to 86 patients with symptoms. They will receive infusions of the cell therapy, in the hopes it will prevent them from developing the more severe form of the disease, Dr. Robert Hariri, Celularity’s founder and chief executive, said in an interview Wednesday.

“The objective here is preventative,” Dr. Hariri said. “If the timing of giving this can prevent those patients who have early disease from progressing to the more serious, life-threatening form, it could be a very, very useful tool.”

The therapy involves using stem cells from the placenta — known as “natural killer” cells — that help protect a developing fetus or newborn from viruses that have infected the mother. Celularity has been testing these cells in cancer patients.

Dr. Hariri said the trial, which would not include a placebo control group, will take place at academic medical centers around the country. He said the company expected to see initial results about 30 to 60 days after the first patients receive their dose. If this study is successful, Dr. Hariri said, the company would move to a placebo-controlled study that would evaluate the drug’s efficacy against the disease.

At least one outside expert said the approach could present safety risks. Paul Knoepfler, a stem cell researcher at the University of California, Davis, said that patients with coronavirus can develop severe reactions where their immune systems go too far in attacking cells in their lungs, causing damaging inflammation. Other cell therapies tested in China are designed to dampen the immune response. He said one risk with the natural killer cells is they could go in the other direction, exacerbating respiratory problems “by massive killing of the patients’ respiratory cells.”

Despite the scant evidence, Mr. Giuliani has become an early booster, interviewing Dr. Hariri on a podcast published on his website Saturday, and praising the treatment on Twitter, saying, “this therapy has real potential.” In a tweet on Saturday, he added, “Let’s hope F.D.A. can recognize that their cumbersome process designed to keep us safer, if it is not altered dramatically in times of great need, can result in unimaginable loss of human life.”

Around the same time, Twitter deleted a post by Mr. Giuliani that it said violated its rules. The tweet, from March 27, made unfounded claims about the malaria drug hydroxychloroquine, one of the treatments that Mr. Trump has supported.

Dr. Hariri said that he has known Mr. Giuliani for years and that the appearance on his podcast was “a friendly chat between people who know each other and who share a common interest in this particular response to this disease.”

He said that he has no business relationship with Mr. Giuliani, and that Mr. Giuliani is not representing him in any way, either paid or unpaid. “I don’t have anything to do with what the mayor tweets or whatnot, and I don’t agree or disagree with anything,” he said.

Dr. Hariri said the company would follow the established process for testing whether a drug works. “We have waited for the F.D.A. to complete their review, which they did in a heroic and quick fashion,” he said.

On Wednesday evening — the same day the F.D.A. approved his trial — Dr. Hariri praised the appearance by the agency’s commissioner, Dr. Stephen Hahn, on the conservative Fox News talk show “The Ingraham Angle.”

“We are fortunate to have Dr. Hahn at the helm,” he tweeted.