Basophils from patients with rheumatoid arthritis (RA) respond to RNA, DNA and immune complexes (aggregated IgG) with histamine release. The RNA response was well correlated to the clinical activity of the disease, since histamine liberation was found in all patients with severe activity, whereas no liberation was observed in patients with moderate or quiescent activity. A less significant correlation was obtained with DNA and aggregated IgG. In contrast, no response was obtained with RNA, DNA and aggregated IgG in patients with systemic lupus erythematosus (SLE) or in controls. In the RA and the SLE groups no significant correlation was found between the response of RNA, DNA and aggregated IgG and the serum titres of anti-DNA and antinuclear antibodies. No difference in basophil cell count in peripheral blood and basophil histamine content was found between RA, SLE and controls. Our results point to an involvement of an autoimmune type I reaction in the pathogenesis of RA directed against the nuclear components RNA and DNA and against immune complexes.