Peripheral T lymphocytes from 31 patients with monoclonal gammopathy of undetermined significance (MGUS), and from a group of controls of the same age range, were stained using monoclonal antibodies of the OKT series. The absolute number and the percentage of OKT3+ cells did not differ in patients compared with the controls. The percentage and absolute number of T-cell subsets with helper/inducer OKT4+ and suppressor/cytotoxic OKT8+ phenotype were not different from those of the controls, thus the OKT4/OKT8 ratio in the patients with MGUS was normal (1.60 versus 1.57 in normal controls). These results suggest that MGUS is a B-cell disorder without imbalance of peripheral T-cell subsets unlike B-cell malignancies such as multiple myeloma and B-cell chronic lymphocytic leukemia.