Neutrophilic granulocytes (PMN) are attracted to sites of inflammation by chemotactic factors, the most potent of which are the complement split product C5a, the leukotriene B4 and the bacterial chemotactic factor-related tripeptide formyl-methionyl-leucyl-phenylalanine (FMLP). In addition to inducing directed migration, these agents increase the adherence of PMN to synthetic surfaces and endothelial cells; some stimulate an oxidative burst and the production of reactive oxygen derivatives, and they may be involved in the release of granule constituents. Here, we describe studies on the activities stimulated by a novel monocyte-derived chemotaxin (MOC). Human MOC attracted human PMN, but not monocytes or eosinophils. Like all chemotactic agents, it increased the adherence of PMN on nylon fibers. In contrast to other chemotactic factors it did not stimulate the release of superoxide anion regardless whether the cells were in suspension or adherent on nylon fibers. There was no release of the primary granule enzyme glucosaminidase or the secondary granule component vitamin B12-binding protein in the absence or presence of cytochalasin B. The results suggest that MOC is a unique chemotactic agent with properties different from the most potent chemotactic factors C5a, LTB4 and FMLP. The delayed release from macrophages suggests its involvement in protracted and chronic inflammatory reactions.