A simple and reliable analytical procedure was developed for determination of platelet-activating factor (PAF) in human plasma using radioimmunoassay (RIA). The assay system consisted of lipid extraction with 2-propanol, lipid separation by Amprep octadecyl minicolumn chromatography and thin-layer chromatography and RIA (charcoal method), and was suitable for quantitation of 30-1000 pg of PAF. The sensitivity of RIA for PAF was notably higher than that for sn-2-short-chain PAF-like phosphatidylcholines. This assay system was then applied for measurement of PAF in human plasma. The normal level of plasma PAF was 54 +/- 40 pg/ml (n = 35), whereas plasma PAF levels in patients with liver cirrhosis (LC) and disseminated intravascular coagulation (DIC) were significantly elevated to 238 +/- 314 pg/ml (n = 14) and 591 +/- 328 pg/ml (n = 14), respectively. The values obtained using this assay system were comparable to those obtained by gas chromatography/mass spectrometry analysis and bioassay. These results indicate that our new assay system is useful for determining changes in the level of plasma PAF associated with diseases such as LC and DIC.