cellular and humoral components. Among humoral factors, there is less information on soluble molecules able to act as signals during the immune response (i.e. cytokines). To date, only a few cytokines have been observed in different arthropod taxa such as insects (Spätzle and Unpaired [Upd]-3) and crustaceans (astakine). As it has been proposed that a possible involvement of chemotactic factors may increase the expression of upd-3, we have studied the effects of human recombinant ilnterleukin-8 (hrIL-8) on the immune functions of Drosophila melanogaster SL2 macrophage-like cells. We have found that hrIL-8 enhances the expression of upd-3 and of the putative cytokine, Drosophila helical factor (dhf). Furthermore, hrIL-8 promotes the transcription of the antimicrobial peptide genes defensin and cecropin A1. Beside these evidences on humoral factors, hrIL-8 also increases the phagocytic activity of SL2 cells. Our data suggest the existence in D. melanogaster of one or more soluble factors that possibly share some structural similarity with IL-8, eliciting an immune response involving simultaneously cellular and humoral components.