Abstract: In the present study the effect of platelet activating factor (PAF) antagonist WEB 2086 on the production of tumor necrosis factor (TNF) from primed murine peritoneal macrophages was investigated. At 10_-6 and 10_-5 mol/L, WEB 2086 was found to significantly inhibit LPS induced TNF production from macrophages by activated thioglycollate solution. WEB 2086 inhibition of TNF release began 4 h after LPS stimulation and lasted 22 h with a peek at 16 h. The results showed that PAF might play an important role in the production of TNF. Four methods were compared in the bioassay of TNF. The data demonstrated that L-929 cells treated with actinomycin D and sodium fluoride were the most sensitive for the assay of TNF. This method was employed in this study.