Spontaneous IgG and IgG subclass secretion patterns by isolated intestinal mononuclear cells (MNC) from control and inflammatory bowel disease (IBD) specimens were examined. Intestinal MNC from IBD specimens spontaneously secreted more total IgG than did control intestinal MNC. This increased spontaneous IgG secretion by ulcerative colitis intestinal MNC was primarily due to markedly increased production of IgG1. Slightly increased secretion of IgG3, but not IgG2 by ulcerative colitis intestinal MNC was present when compared with control and Crohn's disease intestinal MNC. In contrast, Crohn's disease intestinal MNC exhibited increased spontaneous secretion of all the IgG subclasses examined, with IgG2 being predominant.