The vav proto-oncogene product (p95vav) is specifically expressed in cells of the hematopoietic system, contains one Src homology 2 and two Src homology 3 domains, and is a substrate for receptor and non-receptor tyrosine kinases. Immunoblotting experiments using an anti-phosphotyrosine monoclonal antibody showed that interferon alpha (IFN alpha) induces rapid tyrosine phosphorylation of p95vav after binding to its cell surface receptor in the U-266 human myeloma cell line. The IFN alpha-induced tyrosine phosphorylation of p95vav was time- and dose-dependent, confirming the specificity of the process. IFN alpha-dependent tyrosine phosphorylation of p95vav was also observed in other hematopoietic cell lines of B-cell origin (Daudi), T-cell origin (MOLT-4), and promyelocytic origin (HL-60). Immunoprecipitation experiments performed with 32P-labeled U-266 cells and phosphoaminoacid analysis of the bands corresponding to p95vav showed that p95vav is phosphorylated on serine residues prior to IFN alpha stimulation of the cells. After IFN alpha stimulation significant amounts of phosphorylation of p95vav on tyrosine residues were detectable. Tyrosine phosphorylation of p95vav in U-266 and HL-60 cells was also induced by two other Type I IFNs, IFN beta and IFN omega. Altogether these data suggest that the vav proto-oncogene product is a substrate for a Type I IFN-regulated tyrosine kinase(s) and may be involved in the signal transduction pathway of Type I IFNs in hematopoietic cells.