Leukotriene B4 (LTB4) has been shown to affect several interleukin (IL)-linked functions of human lymphocytes. In this study, we investigated whether LTB4 regulates IL-5 generation from human T cells and subsequently modulates eosinophil functions. Preincubation of T cells with very low concentrations (10(-12) to 10(-8) M) of LTB4 induced concentration-dependent IL-5 production, the event occurring after the first 24 h of cultivation. However, direct action of LTB4 to IL-5 generation is strictly dependent on a preincubation with appropriate concentration of LTB4. In contrast, the stereoisomer of LTB4, 5S,12S-dihydroxy-6,8,10,14-eicosatetraenoic acid showed no enhancement of IL-5 production. IL-5 released from LTB4-primed T cells elicited sustained viability of mature eosinophils and reduced the content of eosinophil cationic protein in their crystalloid matrix by degranulation. These data suggest that LTB4 induces bioactive IL-5 production from T cells and that the released IL-5 modulates eosinophil functions which might play a crucial role in eosinophil-linked allergic inflammatory process.