The immunoglobulin constituents of hypertrophied lymphoid nodules in the intestinal tracts of six patients with the variable immunodeficiency syndrome and one patient with selective IgA deficiency were evaluated by the peroxidase-labeled antibody technique. The nodules were found to contain a dense population of IgM-bearing lymphocytes and much intercellular IgM. Evidence that the cells were engaged in IgM synthesis was the presence of the immunoglobulin in the perinuclear spaces and endoplasmic reticulum. Most of the IgM lymphocytes also had surface membrane IgM, and both kappa and lambda light chains were found in lymphocytes of individual nodules. Only a few cells containing IgD, IgG, or J chain, and none containing IgA, were found. We conclude that the intestinal lymphoid nodules associated with hypogammaglobulinemic states are populated principally by IgM B-lymphocytes of polyclonal origin.