Numerous efficacy studies in rodents revealed that 1,4-phenylenebis(methylene)-selenocyanat (p-XSC) is a more effective chemopreventive organoselenium compound and less toxic than benzyl selenocyanate (BSC) or the inorganic compound Na2SeO3. To explore mechanisms which mediate chemopreventive activities of p-XSC we have tested its effect on protein kinase A and C using in vitro and cell culture systems. While p-XSC did completely inhibit PKC and PKA activity, BSC was less active and Na2SeO3 had no effect. Comparative EC,, revealed values of 0.1, 1 and > 10 mu M for p-XSC, BSC and Na2SeO3, respectively. p-XSC was also capable of inhibiting protein phosphorylation in cultures of primary human fibroblasts and altered morphology of rat fibroblast (R6) cells. When combined, sub-optimal doses of p-XSC and staurosporine yielded an additive effect on cell morphology. The ability of p-XSC and BSC to inhibit protein kinase A and C activities may in part account for the mechanism(s) by which these agents mediate their chemopreventive effects.