Peripheral blood lymphocytes from 17 patients with autism were separated on a Ficoll-Hypaque density gradient. Patients had normal numbers of T and B cells and T cell subsets. Although CD4:CD8 ratios were normal for the whole group (2.09 +/- 0.97), 6 patients had elevated ratios (> 2.2) and 5 had decreased ratios (< 1.5). Mitogen-induced proliferation (concanavalin-A and phytohemagglutinin) was normal as was the autologous mixed lymphocyte reaction for the whole group. There was an abnormally increased percentage of DR+ (activated) T lymphocytes in 11 patients. With increasing age percentage of DR+ lymphocytes decreased. No patient had interleukin-2 (IL-2) receptor+ cells. Similar investigations performed on blood samples from 8 girls with Rett syndrome produced normal results. 11 of 17 autistic patients had an abnormally increased percentage of DR+ but not IL-2 receptor+ lymphocytes suggesting 'incomplete' activation, a finding which is seen in autoimmune diseases. The decrease in activated cells with increasing age suggests that there may be an autoimmune process which is more active earlier in life in a subset of autistics.