A B S T R A C T The human inducer (T4+) and reciprocal cytotoxic/suppressor (T5+/T8+) subsets have been defined by monoclonal antibodies. In the present study, we examined the relationship of naturally occurringanti-T cell autoantibodies found in patients with active juvenile rheumatoid arthritis (JRA) to these subsets. In one approach, normal T cells were treated with anti-T4 or anti-T8 to eliminate the corresponding subset of cells and then analyzed for reactivity with JRA sera. It was found that JRA sera were reactive with only 15% of an enriched cytotoxic/suppressor population, whereas they reacted with 37% of an enriched inducer population. In reciprocal studies, JRA+ T cells were eliminated with JRA sera and complement and the residual T cells (JRA-) reacted with monoclonal antibodies and indirect immunofluorescence on a fluorescence-activated cell sorter. As expected, the JRA sera and complement treatment of unfractionated T cells markedly diminished the T4+ subset, whereas there was a concomitant increase in T cells reactive with anti-T5 and anti-T8. A similar diminution in T4+ T cells was found in the circulating peripheral T cell compartment of patients with active JRA who possessed the JRA antibody.Functional studies demonstrated that removal of the JRA+ population of T cells diminished phytohemagglutinin and soluble antigen proliferative responses,