Peripheral blood lymphocytes from 16 patients with juvenile rheumatoid arthritis and 27 age-matched healthy controls have been studied, using several lymphocyte markers: electrophoretic mobility (EM), E-rosettes, immunofluorescence, and refringency. Eight patients (mean age, 6 years) were selected with a typical EM pattern--that is, a decrease in the mean EM of T cells and increase in B versus T-cell ratio. The other group of patients (mean age, 11 years) showed no significant difference when compared with their age-matched controls, with the exception of the positive refringence test. These findings suggest an impairment in the maturation of the immune system in childhood, which in turn may be responsible for the increased susceptibility to disease.