DMBA induced rat mammary tumors were used to study the association of tyrosine protein kinase activity with tumor growth. Pharmacological manipulations of blood prolactin level, by perphenazine and bromocriptine, were used to stimulate or arrest tumor growth, respectively. During perphenazine treatment, a 2-3-fold increase in membranal tyrosine protein kinase activity, measured with angiotensin II as substrate, preceded the 3-4-fold increase in tumor area. At the same time the cAMP-dependent protein kinase activity, measured with kemptide as substrate, did not change. Tyrosine protein kinase Mammary tumor DMBA Tumor growth.