Neoplastic cells of 18 patients with multiple myeloma were studied using a panel of 6 monoclonal antibodies to B cells and monospecific antisera against the light chain types of immunoglobulin. OKT10 bound to the myeloma cells of all the patients, although only a small percentage of the cells reacted in 3 instances. Monoclonal slg was present in 5 patients. HLA-DR antigen detected with OKIa-1 was found in 5 patients. B1 bound to a small percentage of the myeloma cells only in 2 patients who had received treatment. The B1+ cells were always sIg+. In 3 patients, BA-2 reacted with the myeloma cells. BA-1 and BA-3 invariably did not react with the cells. Myeloma cells with B cell markers were found more frequently in treated patients than in untreated patients. The RPMI8226 line was also studied and found to react with OKT10 and BA-2. The results of this study show the presence of phenotypic variety in myeloma cells.