Analogues of staurosporine were synthesized and their ability to inhibit protein kinases was examined. Staurosporine is a potent but non-selective inhibitor of in vitro protein kinase C (PKC) activity (K5, 6.0 nn). The derivative CGP 41 25 I had reduced PKC activity with an IC,, of 50 nn but showed a high degree of selectivity when assayed for inhibition of cyclic AMP-dependent protein kinase (IC5, 2.4 p ~) , S6 kinase (IC5, 5.0 p ~) and tyrosine-kinase-specific activity of epidermal growth factor receptor (lC.so. 3.0 p ~) . Staurosporine and CGP 41 251 exerted growth inhibition in the human bladder carcinoma line T-24, human promyelocytic leukemia line HL-60 and bovine corneal endothelial cells at concentrations which correlated well with in vitro PKC inhibition. In addition, both compounds inhibited the release of H,O, from human monocytes pre-treated with 12-0-tetradecanoyl-phorbol-13acetate at non-toxic concentrations. In vivo anti-tumor activity was examined in T-24 human bladder carcinoma xenografts in athymic nude mice. Tumor growth inhibition tests revealed significant anti-tumor activity (2p < 0.001) at 1/10 of the maximum tolerated doses for both compounds. By contrast, a closely related derivative of staurosporine (CGP 42 700) was inactive at concentrations of over 100 pn in all in vitro enzyme and anti-proliferative assays as well as in animal tumor models. Our data suggest an association between PKC inhibition and anti-proliferative and anti-tumor activity.