Stem cells are cells that can develop into a variety of specialized forms, like tissue or blood. Many of these clinics offer treatments billed as novel stem cell therapies that are derived from cells in the blood, fat or birth tissue.

While legitimate research on stem cells has tremendous promise, some “bad actors” are trying to cash in on patients’ hopes with “purported therapies,” according to a statement by Dr. Scott Gottlieb, the F.D.A. commissioner, and Dr. Peter Marks, director of the agency’s biologics center. The proliferation of rogue clinics has long been a concern of Dr. Gottlieb, who announced his resignation last month and will leave his post on Friday.

In the past year, the statement said, the F.D.A. has sent “regulatory correspondence” to 45 manufacturers and health care providers that market stem cells. The agency also said two lawsuits were pending. One is against U.S. Stem Cell Clinic of Sunrise, Fla., where three patients lost their sight after having stem cells injected into their eyes.

The other lawsuit is against the California Stem Cell Treatment Center, with locations in Rancho Mirage and Beverly Hills, and the Cell Surgical Network Corporation of Rancho Mirage, which treated cancer patients with a stem cell product made with smallpox vaccine. The F.D.A. said the product was risky.

The latest warning letter, five pages long, went to Syed M. Raheel, identified by the F.D.A. as the president and owner of Cord for Life, in Altamonte Springs, Fla. The agency says the company makes unapproved products from umbilical cord blood and sells them in platinum, gold and silver categories. The letter stated that the company had violated federal laws by not obtaining F.D.A. permission to market its products.