The effect of macrophages on spontaneous immunoglobulin production by isolated human intestinal mononuclear cells (MNC) is unknown. Depletion of macrophages by adherence to fibronectin or by panning with macrophage-specific monoclonal antibody 3C10 lead to a significant reduction in IgA. IgG and IgM production by intestinal MNC from both normal (n = 10) and inflammatory bowel disease (IBD) (n = 13) mucosa. The reduction in immunoglobulin produced by macrophage-depleted intestinal MNC was greater in IBD patients than in normal controls. There was a significant correlation (r = 0.816, P less than 0.001) between the percentage of macrophages depleted by panning with 3C10 and the reduction in IgG produced by macrophage-depleted intestinal MNC. Addition of either fibronectin-adherent cells or the supernatant from these macrophage-enriched cells enhanced immunoglobulin production in a dose-dependent fashion. A greater increase in IgG production by macrophage-depleted cells was seen when cultured with supernatant from inflamed IBD mucosal cells, compared with that from normal mucosal cells. The soluble factor(s) responsible in the supernatant was acid and heat susceptible but was not affected by freezing and thawing. Addition of recombinant human interleukin-1 beta or human interferon-gamma to cell cultures did not influence immunoglobulin production. Thus, human intestinal macrophages enhance spontaneous immunoglobulin production by isolated intestinal MNC by secreting soluble factor(s) which remain to be fully characterized.