Human polymorphonuclear leucocytes (PMN) incubated with surface-bound immune complexes (IC) release a substance that induces platelet aggregation and serotonin-release. This substance was identified as platelet-activating factor (PAF) on the basis of its sensitivity to phospholipase A2 and of its purification by thin-layer chromatography in identical conditions to those used to purify zymosan-induced PAF. We used two types of substrates to absorb our IC: Sepharose particles to which we coupled human serum albumin, and which were later incubated with specific rabbit antiserum to form surface-bound immune complexes, and human erythrocytes, to which soluble IC can be passively adsorbed. Both types of surface-bound IC were found to stimulate the release of PAF by human PMN in the absence of complement. These results suggest that PMN may play a central role in the early stages of IC-induced inflammation: they recognize IC adsorbed to red cells or to any other cell able to adsorb IC, and they induce the activation of platelets and release of vasoactive amines, which leads to the increase of vascular permeability believed to be essential for extravascular IC deposition.