Peripheral blood cells from untransfused rabbits that were treated with a moderate dose of benzene during 2 months were cocultivated with autologous and normal bone marrow. In a significant number of experiments, CFUE and BFUE growth of both autologous and normal bone marrow was suppressed. Further experiments showed that this inhibition was associated with adherent mononuclear peripheral blood cells. It is concluded that benzene, an established myelotoxic agent, also induces CFUE and BFUE inhibiting activity in adherent peripheral blood cells.