A panel of monoclonal antibodies has been used to identify peripheral blood lymphocyte subpopulations in 13 patients with mycosis fungoides and in 13 normal subjects. An increase in the T6+ population and a decrease in the T8+ population were noticed in our patients. Their T4/T8 ratio was considerably augmented. In 4 patients reevaluated after treatment, marked changes in the relative proportions of T lymphocyte subsets had occurred. The possible pathogenetic significance of these immunological alterations is discussed.