To assess the relative contribution of different leucocyte subpopulations to LTB4 production, peripheral blood leucocytes from human donors were separated into polymorphonuclear neutrophils (PMN) and lymphocytes/monocytes (L/M) and were then stimulated in vitro with the Ca-ionophore A 23187 for different times. The supernatants were analysed for their contents of leukotriene B4 (LTB4) and its omega-metabolites by HPLC-analysis and column fractions were also examined for their chemotactic activities towards eosinophils in vitro. PMN supernatants contained greater quantities of LTB4, 20-OH-LTB4, 20-COOH-LTB4, and chemotactic activities than did L/M supernatants. On the other hand, the time dependent decrease of LTB4 and chemotactic activity and the increase of omega-metabolites were higher in PMN than in L/M. These results would correlate with the greater role of PMN in acute and that of monocytes in chronic inflammation.