In previous studies bryostatin has been shown to cause dosedependent stimulatory or inhibitory effects on colony formation in acute myeloid leukemias. In this study we investigated the inhibitory effect of high dose bryostatin-1 (bryo-1) on normal human bone marrow mononuclear cells (BMNC) colonyforming capacity. Preculturing BMNC for 24 h with 250 nM bryo-1 reduced colony formation by 66 ± 11% whereas this treatment did not reduce clonogenic capacity of highly purified CD34