Leukotriene B4 (LTB4), one of 5-lipoxygenase (5-LO) products of arachidonic acid, was reported to be more potent than leukotriene B5 (LTB5), one of 5-LO products of eicosapentaenoic acid, in the activation of neutrophil functions through the differential potency between these leukotrienes in calcium mobilization. So we compared the effect of LTB4 and LTB5 on the induction of interleukin 1 (IL-1)-like activity and calcium mobilization in human blood monocytes. LTB4 significantly augmented IL-1-like activity in monocytes, however, LTB5 had no significant effect. Also LTB4 was apparently more potent than LTB5 in inducing calcium mobilization in fura-2-loaded monocytes. IL-1 production in monocytes was reported to be partly dependent on cytosolic free calcium. These results, therefore, may indicate that the different activity between LTB4 and LTB5 in the enhancement of IL-1-like activity could be partly ascribed to the different potency in inducing calcium mobilization between LTB4 and LTB5 in human blood monocytes.