B-cell function in vitro was studied in patients with natural rubella virus infection. Lymphocyte responses to mitogens in a blast transformation test were decreased at the onset of the rash. Spontaneous immunoglobulin production by peripheral blood lymphocytes was increased during the acute phase of illness. In regulatory T-cell subpopulations, increased proportions of cells with suppressor/cytotoxic phenotypes and decreased proportions of cells with helper/inducer phenotypes were detected simultaneously with the increased spontaneous immunoglobulin production. All parameters studied were recovered during the convalescent phase.