An application of the particle beam-liquid chromatography-mass spectrometry technique to the quantification of hydroxyeicosatetraenoic acids (15-, 12-, and 5-HETE) in biological samples is presented. The acids are extracted with Ethyl acetate and then transformed into pentafluorobenzyl esters, thus increasing the sensitivity of their detection by negative chemical ionization mass spectrometry. Reverse-phase HPLC separation of HETEs is performed in about 10 min with a water-methanol gradient. The procedure shows a detection limit of nearly 0.5 pmol, about one order of magnitude lower than that of the widely used HPLC/UV methods. The quantitative determination is linear (r2 greater than 0.998) for all of the HETEs in the range tested (3-1500 pmol) and a CV lower than 8.5% was observed for repeated analysis of samples. As an application of the method, HETEs formed from endogenous arachidonate were evaluated in extracts obtained from coincubates of platelets and neutrophils stimulated with calcium ionophore A23187.