Peripheral blood mononuclear cells from 25 patients with measles and 13 patients with other diseases from Lima, Peru, were studied by immunocytochemical staining for cell surface antigens indicating the type of cell (Leu 4, Leu 3, T8, B1, M1, or esterase) and the state of cell activation (T10 and IL-2 receptor). Measles patients were studied during the first 2 weeks of disease and had no alteration in the proportion of cells which were positive for any subset marker or in the ratio of helper/inducer to cytotoxic/suppressor T cells compared to controls. Measles patients, however, had a greater number of cells expressing the activation antigens T10 and IL-2 receptor than controls. Incorporation of [3H]thymidine was also higher in measles patients when exogenous natural or recombinant IL-2 was added to unstimulated cultured cells. We conclude that the peripheral blood mononuclear leukocytes of patients with measles have normal proportions of helper/inducer and cytotoxic/suppressor T lymphocytes, B lymphocytes, and monocytes but that an increased number of these cells are in an activated state.