Human polymorphonuclear granulocytes (PMNs) synthesize leukotriene B4 (LTB4) as a response of cell activation. Inactivation of the potent inflammatory mediator proceeds via omega-oxidation, resulting in the formation of 20-hydroxy- and 20-carboxy-LTB4. The main metabolite after stimulation with the chemotactic peptide N-formyl-methionyl-leucyl-phenylalanine (FMLP) is 20-carboxy-LTB4, and after stimulation with the calcium ionophore A23187 is 20-hydroxy-LTB4. Differences in the LTB4 inactivation pathway were also observed when the catabolism of exogenously added LTB4 was analysed. In contrast to resting cells or cells preactivated with FMLP, prestimulation with the ionophore or with phorbol esters resulted in the inhibition of 20-carboxy-LTB4-generation. This decrease correlated with the reduction in specific [3H] LTB4-receptor expression. Studies with the non-penetrating diazonium salt of sulphanilic acid, which is known to interact with ectoenzymes, revealed that LTB4 is metabolized via receptor-mediated uptake. Our data suggest that the reduction in the amount of LTB4-receptor sites inhibits the conversion of 20-OH-LTB4 into 20-COOH-LTB4.