The human proerythroblastic leukemia cell-line K562 was induced to differentiate into megakaryocytic cells by 12-O-tetradecanoylphorbol-13-acetate (TPA). Megakaryocytic differentiation was detected when lineage-specific monoclonal antibodies were used to monitor the effect of TPA on K562 cells. A monoclonal anti-platelet antibody (C17) directed against an epitope present on GP IIIa appeared to react with K562 cells after induction. This was observed together with the disappearance of glycophorin A, the erythrocyte-specific lineage antigen. The induced megakaryocytic cells were also detected by ultrastructural platelet peroxidase (PPO). Immunoprecipitation, after ectolabeling of the cells with the C17 antibody and SDS-polyacrylamide gel electrophoresis, proved that TPA-induced K562 expressed both GP IIIa and GP IIb. However, the monoclonal antibody C15 directed against another epitope of platelet GP IIIa reacted only partially, or not at all, indicating that GP IIIa expressed on TPA-induced K562 differs structurally from that on normal platelets. K562 clones, expressing glycophorin A in all cells, were obtained by limiting dilution and culture. When these clones were treated with TPA, again megakaryocytic cells were obtained. These findings are discussed in relation to normal megakaryocytopoiesis.