Patients with systemic lupus erythematosus (SLE) frequently have anti-lymphocyte autoantibodies, some of which also bind to surfaces of neurons. Since anti-ribosomal P protein autoantibodies (anti-P) from SLE patients also bind to surfaces of neurons, we hypothesized that anti-P are anti-lymphocyte antibodies. A panel of human T lymphocytes was evaluated for anti-P binding by indirect immunofluorescence. Affinity-purified anti-ribosomal antibodies were used as a source of anti-P. These autoantibodies bound to the surfaces of all transformed T cell lines tested. This binding was not mediated by Fc receptors. It was inhibitable by ribosomes. Anti-P bound to circulating T lymphocytes from healthy adults and children. They also bound to thymocytes and cord blood T cells from normal neonates. Circulating T cells from SLE patients with anti-P bound less anti-P than cells from healthy controls. Two patients were studied on multiple occasions. The capacity of their T cells to bind anti-P correlated inversely with titres of anti-ribosomal antibodies. Anti-ribosomal antibodies, other than anti-P, also appear to bind to T cells. The surface of T cells contains a protein with the size and antigenicity of the ribosomal P protein, P0. We conclude that anti-ribosomal antibodies are a subset of anti-lymphocyte autoantibodies. Their possible role in the pathogenesis of lymphopenia or lymphocyte dysfunction in SLE has to be defined in further studies.