The activity of 1-phosphatidylinositol 4-kinase (EC 2.7.1.67), the first committed ATP-utilizing enzyme of inositol 1,4,5-trisphosphate and diacylglycerol biosynthesis, was determined in a spectrum of rat hepatomas of different growth rates, in sarcoma, and in normal tissues of high cell renewal rates which include differentiating and regenerating liver. A standard isotopic method was developed to measure the enzymic activity in crude particulate extracts. In this assay, the enzyme activity was linear with time for 2 min and proportional with protein concentrations over a range of 0.1 to 1.0 mg per 0.1 ml reaction mixture. The optimum pH for both liver and hepatoma enzyme was 7.4. The apparent Km values of the kinase for ATP, Mg2+, and the substrate phosphatidylinositol in normal liver were 0.03, 10, and 0.2 mM, respectively, and in rapidly growing hepatoma 3924A 0.01, 0.1, and 5.3 mM. The kinase activity in adult rat livers was 0.3 to 0.5 +/- 0.01 nmol/h/mg protein. In hepatomas of slow and intermediate growth rates, kinase activity increased 5.3- to 7.6-fold, and in rapidly proliferating hepatoma 3924A, it was elevated 28.5-fold over that of normal liver. In rat sarcoma, kinase activity was 13.2-fold higher than in normal muscle. To clarify further the linkage between kinase activity and proliferation, enzymic activity was determined in rapidly growing rat tissues. The kinase activity in rat thymus, bone marrow, spleen, and testis increased 8.4-, 7.6-, 5.6- and 5.6-fold, respectively, over the values of normal rat liver; by contrast, in skeletal muscle, liver, heart, and renal cortex, the activities were low. In the rapidly growing neonatal rat liver and in 24-h regenerating liver, activities were 3.4- and 3.0-fold higher than in the adult resting liver. From this study, the relationship of 1-phosphatidylinositol 4-kinase activity with transformation and cell proliferation is clearly apparent in the markedly increased activity in transplantable hepatomas of different growth rates and in sarcoma and is further emphasized by the high activity observed in newborn and regenerating liver and in thymus, bone marrow, spleen, and testis. Since the kinase activity is linked with proliferation and malignancy, it may well be a sensitive target for chemotherapy.