The biosynthesis of 5(S), 12(R)-dihydroxy- 6,14-cis-8,10-trans-eicosatetraenoic acid, leukotriene B4, by human polymorphonuclear leukocytes was examined in relation to age. The leukotriene B4production by polymorphonuclear leukocytes from unbilical cords, infants, and adults was assayed using high pressure liquid chromatography. The specificity of the leukotriene B4assay was examined by gas chromatography mass spectrometry. Polymorphonuclear leukocytes from 10 umbilical cords, 24 infants and children, and 10 adults were examined for their ability to synthesize leukotriene B4, in vitro after stimulation by the ionophore A23187 or platelet-activating factor. Among the infants and children, there was a slight agedependent increase of leukotriene B4production by polymorphonuclear leukocytes in response to ionophore A23187, but it was not statistically significant. Leukotriene B4production by polymorphonuclear leukocytes in the umbilical cords and infants was not significantly lower than that of polymorphonuclear leukocytes in adults in response to both ionophore A23187 and platelet-activating factor under our experimental conditions.