We have studied the ability of mononuclear cells extracted from the colon (CMC), draining lymph nodes (LNMC), peripheral blood (PBMC) and spleen (SMC) of a patient with ulcerative colitis and severe autoimmune haemolytic anemia to produce red cell autoantibodies. The LNMC, PBMC and SMC secreted immunoglobulin in vitro in pokeweed mitogen culture but not spontaneously. They also produced immunoglobulin when transferred to severe combined immunodeficient (SCID) mice but no anti-red cell activity was demonstrable. In contrast, in vitro cultures of CMC produced immunoglobulin spontaneously, showing the presence of activated B-cells and plasma cells, but anti-red cell activity could not be demonstrated. However, CMC transferred to SCID mice were able to produce IgG with anti-red cell activity. These results concur with clinical observations suggesting that the colon is the source of red cell autoantibodies in these patients.