Leukotriene B4 (LTB4) is metabolized by beta-oxidation, omega-oxidation and the 12-hydroxyeicosanoid dehydrogenase/delta 10-reductase pathway. We have investigated the effects of metabolites formed by the latter pathway on calcium mobilization and migration in human neutrophils and have compared their potencies with those of other LTB4 derivatives. 12-Oxo-LTB4 and 10,11-dihydro-LTB4 were 60 to 100 times less potent than LTB4 in stimulating neutrophils, whereas 10,11-dihydro-12-oxo-LTB4 and 10,11-dihydro-12-epi-LTB4 exhibited still lower potencies. The 6-trans isomers of 12-oxo-LTB4 and 10,11-dihydro-12-oxo-LTB4 were much less potent than the 6-cis compounds. The EC50 values for biologically and chemically (6-cis) synthesized 12-oxo-LTB4 were similar, indicating that the 6,7-double bond is retained in the cis configuration in the biologically formed compound. Methylation of LTB4 markedly reduced its effect on cytosolic calcium levels, whereas addition of a 3-hydroxyl group had a much more modest effect. Modifications of the omega end of the molecule also resulted in lower potencies for calcium mobilization. Nearly all of the compounds tested desensitized neutrophils to LTB4-induced calcium mobilization, which suggests that their effects were mediated by receptors for the latter compound. However, modifications in the carboxyl end of the molecule had smaller effects on desensitization than on calcium mobilization, whereas the reverse was true for modifications in the omega end of the molecule. This suggests that the structural requirements for agonist-induced desensitization to LTB4 may differ to some extent from the requirements for calcium mobilization.