It was recently shown that paf-acether (paf) synthesized in different cell types remains partly cell-associated. In the present work, we tested the hypothesis that cell-associated paf might in fact remain exposed on the external plasma membrane and be able to exert its biological functions. Human polymorphonuclear neutrophils (PMN), stimulated with either opsonized zymosan or ionophore A23187 and then thoroughly washed, induced aggregation of human and rabbit platelets in a time- and dose-dependent manner, whereas no aggregation was observed in the presence of unstimulated cells. Aggregation was inhibited by the specific paf antagonists BN 52021 or WEB 2086. Treatment of stimulated PMN with specific anti-paf antibody before addition to platelets abolished the PMN--paf-mediated aggregation. Microscopic observation of human platelets revealed that aggregates formed by platelets were attached to the neutrophil surface. Paf remained associated with PMN following human PMN-human platelet interaction, in contrast to human PMN-rabbit platelet incubation, where it disappeared from both PMN and platelet surfaces. Our results strongly support the hypothesis that a fraction of cell-associated paf synthesized in neutrophils is located on and/or in the external plasma membrane, where it can act upon other cells by direct cellular contact. Such a mechanism of cell adhesion might play a role in cell physiology (neutrophils but also monocytes/macrophages, eosinophils, and lymphocytes), as well as in the onset and perpetuation of immune and inflammatory reactions.