The distribution of peripheral blood T cells bearing Fc receptors for IgG (T gamma) and IgM (T mu) was determined in 18 patients with cutaneous T-cell lymphoma (Sézary syndrome, mycosis fungoides) and 15 normal controls. The mean percent of T mu and T gamma cells in the patients with morphologically normal lymphocytes was similar to that of the control subjects. In 13 patients with circulating malignant cells representing 30-100% of their peripheral blood lymphocytes, the mean percentages of T mu and T gamma were low. In 9 patients the majority of the malignant T cells did not exhibit Fc receptors for IgG or IgM. In some patients, however, high blood lymphocyte counts resulted in normal or high concentrations of all the T-cell subsets. Further, in 4 patients, lymphocytes with the characteristic nuclear abnormalities of "Sézary/mycosis" cells were found in both the T mu and T gamma, as well as the Tnon gamma non mu, subpopulations. The heterogeneity of the Fc receptors on cutaneous T-cell lymphoma lymphocytes suggests that these receptors are not useful as clonal markers in these disorders.