Production of chemokines (chemotactic cytokines) by neutrophils is likely to be important in the regulation of inflammation and the control of infection. In this study we show that exposure of human neutrophils to various microbial pathogens leads to the production of both macrophage inflammatory protein 1α (MIP-1α) and IL-8. The bacterial microbes, Salmonella typhimurium and Pseudomonas aeruginosa, and Staphylococcus aureus all strongly induced both IL-8 and MIP-1α secretion, whereas Streptococcus pneumoniae, Staphylococcus epidermidis, and the opportunistic yeast Candida albicans were less potent. Saccharomyces cerevisiae and zymosan both induced IL-8 secretion but failed to stimulate that of MIP-1α. Coincubation of neutrophils with the proinflammatory cytokine TNF-α and the micro-organisms also led to differential expression of MIP-1α and IL-8. Significant enhancement of the induction of both MIP-1α and IL-8 by S. typhimurium, P. aeruginosa, and S. pneumoniae as well as by C. albicans was observed. In contrast, while IL-8 production in response to S. cerevisiae and zymosan was enhanced in the presence of TNF-α, no MIP-1α was produced. These combined results indicate that while neutrophils exposed to some micro-organisms alone or in the presence of inflammatory cytokines such as TNF-α will produce both MIP-1α and IL-8, resulting in generation of signals for the recruitment of mononuclear leukocytes and neutrophils, respectively, certain types of micro-organisms can skew this response toward synthesis of IL-8.