The involvement of tyrosine phosphorylation in the early stages of interferon-gamma (IFN gamma)-induced monocytic differentiation of HL-60 cells was studied. Immunoblotting analysis demonstrated that IFN gamma induced rapid changes in the tyrosine phosphorylation of several endogenous cytosolic and membranal proteins. The most prominent of these polypeptides was a 84 kDa protein. In membranes, the IFN gamma-induced phosphorylation of this protein was detectable in 5 min, remained elevated for 3 h and declined thereafter, while a gradual decrease in the phosphotyrosine content was observed in cytosols. In parallel, a 40% increase in the phosphotyrosine phosphatase activity was detected in the later stages of IFN gamma treatment. Rapid changes in tyrosine phosphorylation were detected also in a 64 kDa protein. In contrast, 2-day exposure to IFN gamma was needed to potentiate significantly the tyrosine phosphorylation of a 36 kDa membranal polypeptide. These data support the involvement of tyrosine phosphorylation in the early stages of IFN gamma-induced monocytic differentiation of HL-60 cells.