The macrocyclic lactone bryostatin-1 was found to exert in vitro antineoplastic activity against several leukemic cell lines, including human K562 erythroleukemia, HL60 promyelocytic leukemia, REH and MOLT-4 lymphoblastic leukemias, CCRF-CEM lymphoma, KG-1 myeloid leukemia, and murine P388 lymphocytic leukemia. No statistically significant difference in sensitivity to bryostatin-1 was found between adriamycin-resistant P388 and K526 subclones and their sensitive counterparts. Freshly explanted clonogenic leukemic cells showed a variable sensitivity to bryostatin-1 in 10/12 tested samples. The IC50 of clonogenic leukemic cells was 4 x 10(-3) M bryostatin-1, and that of normal marrow CFU-GM was 10(-5) M. Leukemic cells exposed to bryostatin-1 showed a variable degree of monocytic differentiation as evaluated by ANAE staining and morphology. Bryostatin-1 is also able to inhibit the growth of CFU-GM from myelodysplastic marrow and to shorten the duration of dysplastic hematopoiesis in liquid culture. In conclusion, these data suggest that bryostatin-1 is a potent antileukemic agent in vitro that may be potentially useful for clinical studies.