About 1.5% of phosphorylated amino acid residues of HL-60 promyelocytic leukemia cells are phosphotyrosine. Induction of granulocytic differentiation by exposure to dimethylsulfoxide decreased tyrosine phosphorylation to 0.2%. A maximum 3-fold increase in tyrosine kinase activity and a 7-fold increase in protein phosphotyrosine phosphatase activity accompanied this change. Monocytic differentiation induced by 12-O-tetradecanoylphorbol-13-acetate, caused a decrease in phosphotyrosine levels to 0.1%; tyrosine kinase activity maximally increased 2-fold, and protein phosphotyrosine phosphatase activity increased 11-fold in these differentiated cells. Thus, although total tyrosine kinase activity markedly increased during differentiation, this was counteracted by an even greater elevation in protein phosphotyrosine phosphatase activity. The findings support the concept that tyrosine phosphorylation is important in the regulation of growth and differentiation of leukemia cells.