Monosodium urate crystal (MSU)-induced inflammation was evaluated for increased vascular permeability and platelet activating factor (PAF) production. Intraperitoneal injection of MSU crystals increased vascular permeability and lyso-PAF, as shown by acetylation and bioassay, but PAF was not detected. Intraperitoneal administration of PAF elicited dye-extravasation, which was abolished by the PAF antagonist WEB-2086, but this compound had no significant effect upon MSU-induced dye extravasation. Thus, the presence of lyso-PAF did not reflect in situ production of PAF. Lyso-PAF extracted from normal serum in amounts equivalent to that extravasated could account for the lyso-PAF observed. Thin-layer chromatographic analysis of acetylated extracts from plasma and peritoneal washouts showed identical Rf values to the PAF standard. We conclude that lyso-PAF in the exudate was associated with extravasated plasma proteins rather than a reflection of in situ production of PAF.