The trypanocidal drug, suramin, has been shown to possess antitumour activity, both in vitro and in vivo. Its mechanism of action, however, remains unclear although an effect on signal transduction has been proposed. We therefore studied the in vitro effect of suramin on protein kinase C (PKC), on adenylate cyclase and on the intracellular calcium concentrations [Ca2+]i in human cancer cell lines. Ca(2+)- and phospholipid-dependent PKC was isolated from a normal rat spleen, and compared with that of the human cancer cell lines MCF-7 (breast cancer) and PC3 (prostate cancer). PKC was inhibited by 50% at 55, 40 and 27 microM suramin in the three PKC sources, respectively, while 300 microM of suramin gave 97, 95 and 99% inhibition. With 50 nM staurosporine, a known PKC inhibitor, we observed 80, 99 and 96% inhibition in these three different sources of PKC. Six day exposure of these cell lines to suramin, causing 50% growth inhibition, decreased the Ca(2+)- and phospholipid-dependent PKC activity in MCF-7 cells to 52% of the control and in PC3 cells to 48% at equitoxic concentrations (45 and 150 microM suramin, respectively). These concentrations of suramin slightly increased (approximately 2-fold) the adenylate cyclase activity in MCF-7 cells, but not in PC3 cells. In MCF-7 and PC3 cells, we measured the [Ca2+]i using Fura-2 fluorescence and observed a decrease in MCF-7 cells from 126 to 99 nM when the cells were exposed for 6 days to 45 microM suramin. In PC3 cells, [C2+]i decreased from 131 to 117 nM after exposure to 150 microM suramin. In conclusion, suramin inhibited the Ca(2+)- and phospholipid-dependent PKC activity in both cell lines in a dose-dependent manner. Only in the more sensitive MCF-7 cell line was a significant effect of suramin on intracellular Ca2+ and adenylate cyclase observed, indicating that one of the mechanisms of action of suramin could be mediated by perturbations of intracellular signalling pathways.