Suramin, a synthetic polysulfonated anionic compound, is known to abrogate the activity of a variety of growth factors that serve as ligands for receptor-class protein-tyrosine kinases. Based on this information, we initially hypothesized that suramin treatment would be associated with decreased tyrosine phosphorylation. Upon testing this hypothesis in prostate cancer cell lines, we found that the most conspicuous effect of suramin was to increase the tyrosine phosphorylation of several distinct proteins. Further analyses indicate that suramin-induced increases in tyrosine phosphorylation represent a generalized, but not universal, phenomenon found in cell lines derived from a variety of human tissues. These rapid and specific suramin-induced alterations represent a novel finding for a non-polypeptide pharmaceutical agent and question the hypothesis that suramin exerts its antitumor action simply by abrogation of growth factor action.