Protein tyrosine kinases (PTKs) play a central role in cellular regulation by virtue of their participation in, and control of, signal transduction pathways; they act as a molecular interface between the cell's environment and intercellular metabolism. The mammary gland, unlike most organs, undergoes most of its morphogenesis in juvenile and adult life. The epithelium goes through hormonally controlled cycles of proliferation and regression, the fully differentiated state only being reached at the end of pregnancy. These features make the mammary gland an amenable tissue to study the involvement of PTKs in epithelial cell development and differentiation. We have used a PCR-based molecular cloning strategy to identify PTKs from murine mammary gland cells. Amongst 70 kinase clones characterized we found 3 PTKs previously undescribed in mouse, 4 known PTKs and 5 serine threonine kinases. Expression studies revealed differential tissue specificity and developmental regulation of the 3 previously undescribed PTKs. These results substantiate the view that PTKs are involved in the regulation of cellular differentiation.