The phorbol ester 12-O-tetradecanoylphorbol-13-acetate (TPA) induced megakaryoblastic differentiation and c-sis expression in the human hematopoietic stem cell line K-562. This induction occurred at the transcriptional level, as determined by a nuclear runoff transcriptional assay, and was not a generalized effect of TPA, since the treatment of other hematopoietic cell lines and normal peripheral blood lymphocytes with TPA did not result in the appearance of c-sis mRNA.