Suramin is a prototype of a new class of anticancer drugs. We investigated the action of suramin on the signal transduction pathways to DNA topoisomerase II (Topo II). Suramin showed a growth-inhibitory effect on a human lung cancer cell line (PC-9) with an IC50 of about 160 micrograms/ml. Suramin inhibited the catalytic activity of Topo II with an IC50 of about 100 micrograms/ml without stabilization of the cleavable complex of DNA and Topo II. Suramin decreased the phosphorylation of Topo II with an IC50 of 175 micrograms/ml, but did not change the degree of Topo II expression. These IC50 values for inhibition of catalytic activity and phosphorylation of Topo II were equivalent to the growth-inhibitory dose determined by tetrazolium dye assay. Phosphorylation of the tyrosine residues of Topo II was not changed by suramin. In the presence of okadaic acid, a potent inhibitor of serine/threonine protein phosphatase, suramin also decreased the phosphorylation of Topo II, suggesting that the drug did not act on the serine/threonine protein phosphatases inhibited by okadaic acid. Suramin also inhibited the protein kinase C (PKC) activity of PC-9 cells. These results suggest that suramin decreases the phosphorylation of Topo II mediated by PKC. This effect of suramin might cause the inhibition of Topo II activity resulting in the growth inhibition of tumor cells.