Human granulocyte-macrophage colony-stimulating factor (GM-CSF) is a multipotent cytokine which stimulates the proliferation and differentiation of various lineage of hematopoietic cells. We examined whether GM-CSF stimulates protein phosphorylation in HL60 cells pretreated with differentiation-inducing factors such as PMA, 1,25-$(OH)_{2}VD_{3}$ and DMSO. GM-CSF induced tyrosine phosphorylation of 95 kDa protein in PMA- or 1,25-$(OH)_{2}VD_{3}$ but not in DMSO-pretreated cells. Tyrosine phosphorylation of 95 kDa was detected at 1 min and 2 min after stimulation of GM-CSF in PMA- and 1,25-$(OH)_{2}VD_{3}$-pretreated cells, respectively. Kinase activity which phosphorylates tyrosine residue(s) of the 95 kDa protein appeared to increase in a time dependent manner in PMA-pretreated cells, whereas the expression level of 95 kDa protein was not changed. We also observed that 95 kDa protein was autophosphorylated in immunecomplex kinase assay, suggesting that this 95 kDa protein may be tyrosine kinase which is activated in lineage specific manner. These results suggest that 95 kDa protein may be involved in an early signal transduction pathway of GM-CSF.