The biological effects of leukotriene B4 (LTB4) within the microenvironment are controlled by rapid inactivation. In this regard human granulocytes convert LTB4 into omega-oxidated products (20-OH-LTB4 and 20-COOH-LTB4); moreover, we recently described the formation of unpolar metabolites of LTB4 in human tonsillar and lung macrophages. By means of high performance liquid chromatography (HPLC) we identified the main metabolite of LTB4 as dihydro-LTB4 (5,12-dihydroxyeicosatrienoic acid). Studies on a lymphocyte (74-78%), monocyte (19-22%), and basophil (less than 4%) containing cell fraction isolated from peripheral blood as well as peripheral monocytes purified by elutriation centrifugation revealed evidence that these cells metabolize LTB4 to a very low degree if incubated immediately after isolation. However, after culture for 24-72 h these cells showed a strongly increased capacity to metabolize LTB4. The pattern of metabolites in this cell fraction was identical to bronchoalveolar macrophages (purity greater than 95%). Similarly, the LTB4-reductase was expressed in differentiated human monocytic U-937 cells almost 5-7 h after the addition of dimethylsulfoxide (1.3%) or phorbol-myristate-acetate (16 nM). The expression of this pathway was blocked in the presence of cycloheximide (10 micrograms/ml) whereas actinomycin (3.8 micrograms/ml) had no effects. Dihydro-LTB4 was further metabolized by granulocytes probably via omega-oxidation; therefore, several metabolites could be detected by radioactive high performance liquid chromatography (HPLC) after incubation of bronchoalveolar cells consisting of macrophages and granulocytes with 3H-LTB4. Our data provide evidence for a unique role of macrophages to control the level of LTB4 by generation as well as metabolism into dihydro-LTB4.