The effects of prostaglandin (PG)E1, PGD2 and 9-deoxy-delta 9-PGD2 (PGJ2) on the clonogenic growth of six kinds of human leukemic cell lines (K562, KG1, HL60, U937, THP1 and Molt4) and normal human myeloid progenitor cells (CFU-GM) were studied using semisolid agar cultures. While the degree of suppression of leukemic growth by PGE1 varied from cell line to cell line, PGD2 and PGJ2 equally suppressed the growth of all leukemic cell lines. The potency of growth inhibition was as follows: PGJ2 greater than PGD2 greater than PGE1. The increase of cellular cAMP level induced by prostaglandin treatment did not parallel their cytotoxic potency. Normal myeloid colony formation was also suppressed by PGE1, PGD2 or PGJ2. In contrast to the preferential inhibition of macrophage colony formation by PGE1, such lineage-selective suppression was not observed for PGD2 or PGJ2. These findings suggest that PGD2 and PGJ2 potently inhibit the leukemic growth by a different mechanism from that of PGE1 and by a cAMP-independent mechanism. These prostaglandins seem to be promising chemotherapeutic agents for acute leukemia.