Lymphocytotoxins (LCT) have been recently reported in the serum of patients with inflammatory disease of the bowel, but up to now these antibodies have shown no specificity for B or T lymphocyte subpopulations. A 32-year-old patient with chronic ulcerative colitis, primary hypogammaglobulinemia and a very low number (0.5 to 1.5%) of B lymphocytes in peripheral blood is described. The presence in the serum of a LCT reacting specifically with B cells was demonstrated by cytotoxicity and direct immunofluorescence experiments. Intestinal immunofluorescence studies indicated a dichotomy between blood and gut immunoglobulins, and showed a heterogeneous distribution of plasma cells of the three major classes from the jejunum to the rectum. The significance of the association of hypogammaglobulinemia, chronic ulcerative colitis, and anti-B LCT is discussed. To explain the dissociation between blood and gut immunoglobulins, it is suggested that the intestine was, in this patient, a privileged site for differentiation of B cells.