A case of prolymphocytic leukemia with IgM hypogammaglobulinemia in a 47-year-old man was presented. The leukemic cells possessed Ia-like antigen and receptors for the third component of complement but lacked surface immunoglobulins, cytoplasmic IgM, receptors for sheep red blood cells, or terminal deoxynucleotidyl transferase activity. In vitro immunoglobulin production experiments demonstrated that the leukemic cells did not have the capacity to produce IgM, while patient's T cells were shown to possess helper function on normal B cells to produce immunoglobulins. By these findings, together with the presence of selective IgM hypogammaglobulinemia, it was suggested that the leukemic cells were derived from a B-cell clone of a stage in differentiation and maturation of IgM-forming B-cell spectrum.