A panel of monoclonal antibodies (McAb) from the Wu series was used to characterize lymphocyte subsets of peripheral blood (PB) from 9 patients with multiple myeloma (MM) by means of indirect immunofluorescence assay. In 7 of these patients bone marrow (BM) cells were also studied. Abnormal distribution of T lymphocyte subsets was observed in these patients, and the alterations were quite similar in both PB and BM. A significant increase in the proportion of WUT8+ cells (P less than 0.001) and a reduced WuT4/WuT8 ratio (P less than 0.001) were found in patient group. In both PB and BM of patients, WuT10+ cell counts were significantly higher than normal (P less than 0.001). There was a good correlation between BM WuT10+ cells and BM myeloma cells (r = 0.76, P less than 0.05), whereas no correlation was found between PB WuT10+ cells and BM myeloma cells. The significance of abnormal T-cell subset distribution and elevated WuT10+ cells in MM is discussed.