Paraffin-embedded tissues of 47 cases of multiple myeloma and 4 cases of macroglobulinemia were studied by the immunoperoxidase indirect sandwich technique for identification of intracytoplasmic immunoglobulin. Specific immunoglobulin with a single heavy and/or light chain was readily detected in the neoplastic cells. Studies were performed on iliac crest bone marrow biopsies which had been fixed and decalcified using Zenker's-acetic acid solution and on tissues from various sites which had been fixed in formalin. An excellent correlation was obtained between the specific immunoglobulin detected intracellularly in the plasma cells or lymphoplasmacytic cells and the monoclonal immunoglobulin found in serum or urine of these cases. The immunoperoxidase technique represents a sensitive and specific method for identifying intracellular immunoglobulin in paraffin-embedded tissues.