Follicular dendritic cells (FDC) are able to fix high amounts of immune complexes by C3b or Fc receptors without endocytosis and for long periods of time. In order to determine the function of this retention, we analysed the secretion of prostaglandin E2 (PGE2) by FDC in vitro; indeed, it is well-known that immune complex fixation on cells may induce PGE2 production. FDC were isolated by enzymic digestion of lymph follicles dissected under the biomicroscope from human tonsils or adenoids. Isolated FDC appeared as spherical clusters where they enveloped lymphoid cells with their cytoplasmic extensions. Tests were performed in synthetic culture media or in media supplemented with foetal calf serum. PGE2 production in FDC suspensions was compared to that of lymphocyte or macrophage-enriched populations prepared from the same human tonsils. In all experimental conditions, FDC and macrophage-enriched cell populations produced high levels of PGE2, inversely to lymphoid cell populations. This secretion was inhibited by indomethacin. At the ultrastructural level, we also showed that 3H-arachidonic acid was metabolized in cell membranes of all three cell types. The PGE2 produced in the culture media, according to our experimental conditions, do not influence cell proliferation, as assessed by 3H-thymidine incorporation tests on phytohaemagglutinin-stimulated lymphocytes.