An HPLC-mass spectrometric technique with an ionspray interface was developed for the determination of platelet-activating factor (PAF) and PAF-related compounds in biological samples. HPLC separations were performed using a reversed-phase column. The mass spectra showed intense [M + H]+ ions. Collision-induced dissociation of protonated molecular ions gave characteristic daughter ions corresponding to the phosphorylcholine group. By selective-ion monitoring, a detection limit of 0.3 ng was obtained for all molecules; by multiple reaction monitoring, the same sensitivity was achieved for PAF whereas for lyso-PAF the limit was 3 ng. Finally, PAF was comparatively determined by bioassay and HPLC-MS after extraction from the cell pellets and the supernatants of human polymorphonuclear neutrophils unstimulated or stimulated with opsonized zymosan. The good correlation observed between these techniques indicated the reliability of HPLC-MS for biochemical studies on PAF and PAF-related molecules.