Diflunisal and benoxaprofen (20-100 microM) produced dose-dependent inhibitions of lyso-platelet activating factor: acetyl-CoA acetyltransferase in a lysate of rat pleural neutrophils. Salicylate and aspirin were inhibitory at concentrations of 1 mM and above. Nordihydroguaiaretic acid was a relatively potent inhibitor (I50 = 6 microM). Other compounds, including anti-inflammatory steroids, cyclooxygenase and 5-lipoxygenase inhibitors, appeared ineffective at relevant concentrations. Inhibitions by diflunisal and salicylate occurred at concentrations similar to expected plasma levels in humans at therapeutic doses. An inhibition of platelet-activating factor synthesis may contribute to the antiinflammatory, analgesic, or antipyretic actions of these compounds.