There is no approved treatment for Covid-19, the illness caused by the coronavirus, and researchers and doctors are testing a host of therapies in a desperate bid to save the lives of people who have few other options. President Trump has aggressively promoted two old malaria drugs, which have shown only limited evidence of working as treatments for the coronavirus. He has pushed for the drugs’ broader use in patients without the more rigorous clinical trials typically used to evaluate treatments.

But Mesoblast is taking a more standard approach, testing the cell therapy in 240 patients at more than 20 medical centers around the country, which are part of the Cardiothoracic Surgical Trials Network, a program created by the N.I.H. Patients will be randomly divided into groups that will receive the therapy, and those that will get a placebo. Researchers said the trial could yield initial results within months.

While many people who are infected with the coronavirus experience mild symptoms, others develop a severe case when their immune systems go into overdrive and begin attacking the body’s organs, which is called a “cytokine storm.” This can set off something called acute respiratory distress syndrome, which damages the lungs and is often deadly. Several other treatments are also being tried to calm this storm and reset the immune system.

Dr. Silviu Itescu, chief executive of Mesoblast, said the company decided to test its treatment in these Covid-19 patients because its product had shown good results in children who developed a similar deadly immune reaction called acute graft versus host disease, in which the body’s immune cells can attack healthy cells after receiving a bone-marrow transplant. Their treatment is currently being reviewed by the Food and Drug Administration for use in that disease.

“We put two and two together and said, ‘We think we’ve got something that is safe and could have benefit,’” Dr. Itescu said.

Another stem cell company, Athersys, has said it is also planning a study of stem cells in coronavirus patients with advanced respiratory distress syndrome, but is not as far along.

Nine coronavirus patients at Mount Sinai Hospital in New York have received the Mesoblast treatment on an emergency basis, and doctors there said the initial response was promising. Six patients were removed from ventilation and others were being weaned off or had remained stable — a welcome development when most patients who need ventilator support do not survive.