This study was initiated to investigate the possible involvement of prostaglandin H synthase in the staurosporine-induced prostaglandin production. The time course of prostaglandin H synthase activity in macrophages treated with staurosporine (20 nM) showed that the maximum activity was reached in 20 h. The stimulatory effect of staurosporine on thromboxane B 2 production was maximum at 50 nM staurosporine and this effect was reversed at higher concentrations. Immunoprecipitation of 3 5 S-labeled enzyme using an antibody specific for prostaglandin H synthase-2 paralleled the changes in enzyme activity. These results indicate that increased de novo synthesis of prostaglandin H synthase-2 is largely, if not solely, responsible for staurosporine-stimulated prostaglandin production in macrophages.