LY223982, (E)-5-(3-carboxybenzoyl)-2-((6-(4-methoxyphenyl)-5- hexenyl)oxy)benzenepropanoic acid, is a newly discovered potent inhibitor of specific binding of leukotriene B4 (LTB4) to its receptor on human neutrophils. This study demonstrated that the compound is also a specific antagonist of LTB4-induced neutrophil activation under both in vitro and in vivo conditions. LY223982 was found to be 189-fold more effective in displacing [3H]LTB4 than 35S-N-formyl-L-methionyl-L-leucyl-L-phenylalanine (FMLP) from their corresponding receptors on human neutrophils. The concentration inhibiting 50% of response (IC50) for displacement of [3H]LTB4 (13.2 nM) was only 6.8-fold higher than the value for nonradioactive LTB4. The compound inhibited the aggregation of guinea pig neutrophils caused by LTB4 more strongly than FMLP or platelet-activating factor. The IC50 for inhibition of LTB4-induced responses (74 nM) was 93- and > 135-fold lower than the IC50 for inhibition of the corresponding FMLP and platelet-activating factor-induced effects. LY223982 was also a potent antagonist of the aggregation of human neutrophils by LTB4 (IC50, 100 nM). Chemotaxis of human neutrophils induced by LTB4 was only modestly inhibited by the compound (IC50 = 6 microM) but it had even less effect on cell movement caused by FMLP. LY223982 inhibited transient leukopenia induced in rabbits with LTB4 (ED50, 3 mg/kg) but not with FMLP. It had no agonist activity in any of the test systems. In summary, the results indicate that LY223982 is a potent specific antagonist of LTB4-induced neutrophil activation.