We used an immunoperoxidase technic to detect surface and cytoplasmic immunoglobulin in frozen sections of 46 malignant lymphomas. With the immunoperoxidase technic, differential staining with antisera to the various heavy and light chain classes permitted detection and characterization of monotypic immunoglobulin in frozen sections of 15 of 15 nodular lymphomas and 24 of 31 diffuse lymphomas. The immunoperoxidase technic applied to frozen sections is a convenient and reliable method for the detection of immunoglobulin in lymphoid tissues, which can be performed in a pathology laboratory without the need for special equipment.