Lymph nodes from four cases of mantle zone lymphoma (MZL) were studied immunohistochemically. All the cases had serum M proteins. Immunostaining with paraffin-embedded specimens revealed that proliferating cells of neoplastic nodules possessed the same monoclonal immunoglobulins as those in the serum (IgM, kappa in three and IgM, gamma in one). The plasma cells around neoplastic nodules contained the same monoclonal immunoglobulins as those in the nodules in two cases. In the remainder, most plasma cells around the nodules possessed the same immunoglobulins as those in the nodules. The authors consider these plasma cells to be the cause of paraproteinemia. Immunostaining with paraffin and frozen tissue sections revealed that the neoplastic cells (or neoplastic nodules) were positive for sIgD, sIgM, dendritic reticulum cell antigen (DRC-1), and LN-2 and weakly positive for common acute lymphocytic leukemia antigen (CALLA), and LN-1. These results indicate that the neoplastic cells of MZL have both phenotypes of mantle zone lymphocytes (sIgD, sIgM, and LN-2 positive) and of germinal center cells (sIgM, DRC-1, CALLA, and LN-1 positive). MZL is thus considered to be a neoplasma of B-cells differentiating from mantle zone lymphocytes to germinal center cells.