The in vivo effect of several prostaglandin inhibitory drugs on the hemopoietic stem cell was studied in SJL/J mice. Repeated intramuscular administration of Indomethacin increased the total number of bone marrow (BM) and spleen pluripotent hemopoietic stem cells (CFUS). The number of spleen colonies in lethally-irradiated normal BM recipients, which had been treated with Indomethacin, Aspirin or Flufenamic acid (all prostaglandin inhibitory drugs), was significantly greater than controls' spleen colonies. After exposure of SJL/J mice to different amounts of irradiation, more endogenous spleen colonies and a greater splenic weight were found among mice which had been treated with Indomethacin in comparison to PBS-treated mice. According to our results, treatment with prostaglandin inhibitory drugs stimulates murine hemopoietic stem cell proliferation and enhances stem cell recovery following irradiation.