The effect of human platelet factor 4 (PF4) and beta-thromboglobulin (BTG) on megakaryocyte colony formation in normal subjects as well as in essential thrombocythaemia (ET) and in immune thrombocytopenic purpura (ITP) was studied. Both PF4 and BTG were found to be capable of inhibiting the development of isolated megakaryocytes and their colonies in normal marrow cultures in a dose-dependent fashion. A significant 50% inhibition was seen at a PF4 or BTG concentration of 1-2.5 micrograms/ml, and complete inhibition in the range of 5-10 micrograms PF4 or BTG/ml. The two platelet proteins had similar effects on megakaryocyte development. A combination of PF4 and BTG resulted in an additive effect. Antibodies against PF4 or BTG could effectively neutralize the inhibitory effect of PF4 or BTG respectively. In ET and ITP, in vitro megakaryocyte development was also inhibited by PF4 and BTG in a similar way to that seen in normal subjects, suggesting that the responsiveness of megakaryocyte progenitors to PF4 and BTG is normal in these two disorders. PF4 and BTG did not affect the growth of colony forming units granulocyte-macrophage (CFU-GM) except at very high concentration (greater than or equal to 10 micrograms/ml) but they did inhibit erythroid colony formation by normal and ET burst forming units erythroid (BFU-E). However, the inhibition of BFU-E by PF4 and BTG was dose-related, and a 50% inhibition required a PF4 or BTG dose ranging from 5 to 10 micrograms/ml. These results indicate that PF4 and BTG are involved in negative regulation of normal and pathologic megakaryocytopoiesis and that their inhibition acts predominantly on the megakaryocytic lineage.