We have demonstrated that the 53- and 55-kDa proteins were markedly phosphorylated at tyrosine residues during tetradecanoyl phorbol acetate (TPA)-induced monocytic differentiation of HL-60 cells and that the inhibition of such phosphorylation resulted in suppression of the differentiation. Western blotting, using antiphosphotyrosine antibody PY-20, of antitubulin immunoprecipitate from the lysate of TPA-treated HL-60 cells revealed that tubulin was tyrosine-phosphorylated in TPA-treated HL-60 cells, but not in nontreated HL-60 cells. The tyrosine-phosphorylated 53- and 55-kDa proteins in TPA-treated HL-60 cells were absorbed with antitubulin antibody indicating that the predominantly tyrosine-phosphorylated proteins during the monocytic differentiation were tubulins. By combination of immunoprecipitation and in vitro kinase assay, the protein tyrosine kinases, Fyn and Lyn, which were abundantly induced during monocytic differentiation of HL-60 cells, were demonstrated to be associated with tubulin in the cells. The reorganization of microtubules was observed by immunostaining using rhodamine-conjugated anti-mouse Ig and mouse anti-alpha-tubulin mAb. These data suggest that tyrosine-phosphorylation of tubulin, in which Fyn and Lyn might involve, regulate monocytic differentiation through alteration of microtubule assembly.