Leukotriene B (LTB), a potent lipid chemotactic factor for neutrophils, is 5S,12R-dihydroxy-6,14-cis,8,10-trans-eicosatetraenoic acid (Fig 1), based upon direct comparison of natural LTB with synthetic 5S,12R-dihydroxy-6,8,10,14-eicosatetraenoic acid (5,12-di-HETE) stereoisomers in three biological assays. Of the six synthetic stereoisomers evaluated, only the 5S,12R,6,14-cis,8,10-trans compound had chemotactic potency for human neutrophils in vitro that was comparable to that of natural LTB, with a concentration of 3 X 10(9-9) M eliciting a one-half maximum response. In contrast, the racemic mixture of 5R,12R- and 5S,12S-6,10-trans,8,14-cis, the racemic mixture of 5S,12R- and 5R,12S-6,10-trans,8,14-cis, the 5S,12R-6,8-trans,10,14-cis, the 5S,12R-6,8,10-trans,14-cis, and the 5S,12S-6,8,10-trans,14-cis stereoisomers required concentrations of 3 X 10(-7) to 1 X 10(-6) M to elicit comparable responses. Only natural LTB and its synthetic counterpart elicited a local neutrophil infiltration when injected into the skin of the rhesus monkey at 10 ng and 100 ng per site. Natural and synthetic LTB at a concentration of 3 X 10(-8) M each provoked an EC25 contractile response of guinea pig pulmonary parenchymal strips in vitro, whereas the other four tested stereoisomers of 5,12-di-HETE were inactive at this concentration. Structure-function analyses suggest that the neutrophil chemotactic activity depends critically upon the C-1 to C-12 domain, including the stereochemistry of the 6-,8-,and 10-olefinic bonds and the presence of both hydroxyl groups.