A megakaryoblastic cell line, designated MEG-01, was established from the bone marrow of a patient with blast crisis of Philadelphia (Ph1) chromosome-positive chronic myelogenous leukemia. MEG-01 cells grew in single-cell suspension with a doubling time of 36 to 48 hours. Under the usual culture conditions, approximately half of the cells adhered to the culture flask with extention of pseudopods. MEG-01 cells were positive for the periodic acid-Schiff reaction, alpha-naphthyl acetate esterase, and acid phosphatase, and negative for myeloperoxidase, alpha- naphthyl butyrate esterase, naphthol AS-D chloroacetate esterase, and alkaline phosphatase. Ultrastructural platelet peroxidase was positive in MEG-01 cells. Cytoplasmic factor VIII (FVIII)-related antigen was weakly positive in larger MEG-01 cells by both an indirect immunofluorescent technique with monoclonal antibodies and a direct immunoperoxidase technique using horseradish peroxidase-conjugated conventional rabbit anti-human FVIII antibody. Platelet glycoprotein (GP) IIb/IIIa antigen was uniformly demonstrated on the surface of MEG- 01 cells by both indirect immunofluorescent and immunoperoxidase techniques using antiplatelet GP IIb/IIIa monoclonal antibodies; platelet GP lb antigen was demonstrated only in the cytoplasm of larger MEG-01 cells. MEG-01 cells possessed no markers for B or T lymphocytes or for myeloid cells. Chromosome analysis of this cell line revealed a human male hyperdiploid karyotype with a modal chromosome number of 56 to 58. The Ph1 chromosome was observed in all karyotypes analyzed. This novel human megakaryoblastic cell line may provide a useful model for the study of human megakaryopoiesis and of the biosynthetic mechanisms of proteins unique to megakaryocytic lineage.