Several antiviral compounds showed a toxicity or hemopoietic progenitor cells, 2', 3' Dideoxycytidine (ddc) is the most potent drug in vitro against human T cell lymphotropic viruses and is now being tested on patients with AIDS and AIDS-related complex. We studied its toxicity on mouse hemopoietic progenitors. Results reported here show a toxic activity of ddc but further studies are required to ascertain whether this toxicity is of significance during clinical application.