A case of idiopathic severe aplastic anemia with spontaneous complete remission is described. Hematologic parameters normalized spontaneously 94 days after onset. However, the ability of the patient's bone marrow cells to form granulocytic-macrophagic colony-forming units (CFU-GM) or erythroid burst-forming units (BFU-E) was depressed until the 288th day, in spite of the normalization of blood counts. Incubation of the patient's bone marrow cells with antilymphocytic globulin prior to the culture experiment normalized the number of CFU-GM and BFU-E in correlated studies. Coculture of a target marrow with several concentrations of the patient's lymphocytes or serum resulted in a complete inhibition of CFU-GM. BFU-E growth was inhibited by the patient's lymphocytes, but not by serum, which rather showed burst-promoting activity. The inhibitory effect on target bone marrow persisted for 288 days, when it disappeared concomitantly with the restoration of spontaneous CFU-GM and BFU-E growth.