Precipitating autoantibodies to the URNP particles were used to select 80 patients, and were further characterized by immunoblotting and quantitative ELISA. These immunochemical results have been related to clinical diagnosis, the frequency of nephritis, and Raynaud's phenomenon. Autoantibodies to the 70-kD polypeptide of the U1RNP particle were present in 16 out of 19 patients with mixed connective tissue disease (MCTD) and in 27 out of 61 patients with systemic lupus erythematosus (SLE). The ratio of anti-U1RNP/Sm by ELISA and the frequency of antibody to 70-kD protein were directly related to the frequency of Raynaud's phenomenon and inversely related to the frequency of nephritis.