Platelet activating factor (PAF) is thought to mediate many inflammatory processes and its involvement in health and disease may be clarified by examining PAF levels in human secretions. The known presence of PAF, the ease of obtaining samples and the relative stability of PAF in saliva, makes this fluid a preferred source for examination of PAF in health and disease. The activity of PAF-acetylhydrolase (the PAF degrading enzyme) in saliva was 1,000-fold lower than that found in human plasma. Extraction of saliva with chloroform/methanol/water resulted in 70-90% recovery of PAF. Using the radioimmunoassay (RIA), PAF levels in the range 0.5-21 ng/ml were found in normal human salivas. These values were significantly higher than those reported from bioassay studies based on washed platelets. The validity of the RIA was checked by isolating and quantitating the PAF fraction from whole saliva extract, and by treatment of the extracts with the enzyme phospholipase A2. Direct comparison of salivary PAF levels, determined by both platelet aggregation (PA) and RIA confirmed our original finding that values obtained were lower using the bioassay method. Furthermore, these bioassay values compared favourably with those in the literature. Investigations revealed the presence of a substance(s) in saliva which inhibited PAF-induced platelet aggregation but which did not affect the radioimmunoassay.