The human promyelocytic leukemia cell line HL-60 can be induced to differentiate toward neutrophils and subsequently die via apoptosis in vitro. In this paper, we investigated the roles of protein-tyrosine kinases (PTKs) in retinoic acid (RA)-induced granulocytic differentiation of HL-60 cells. Accompanying the RA-induced differentiation, activities of src family PTKs Lyn and Fgr became detected and reached a plateau 2 days after the stimulation. The immunoblotting using anti-phosphotyrosine antibody (PY-20) showed that the proteins of 56 and 53 kDa were predominantly tyrosine-phosphorylated at day 2. Adsorption and immunoprecipitation of the cell lysate by specific antibodies evidenced that these phosphotyrosine-containing proteins are Lyn and Fgr PTKs. The degree of both activities and tyrosine phosphorylation of these PTKs was reduced to be minimal at day 5 when the HL-60 cells start to die by apoptosis. The inhibitors of PTKs, herbimycin A and genistein, were demonstrated to cause premature cell death of HL-60 cells in the presence of RA. The death was the consequence of an apoptotic process. The Ra-treated HL-60 cells, when incubated with specific c-lyn or c-fgr antisense oligodeoxynucleotide, also underwent premature death at day 2. These data implicate that Lyn and Fgr PTKs prevent programmed cell death to promote granulocytic differentiation of HL-60 cells.