Autoantibodies to small nuclear ribonucleoproteins (snRNP) were studied using the techniques of immunodiffusion, ELISA, and immunoblotting in the sera of 150 patients with systemic lupus erythematosus (SLE), and of 29 patients with mixed connective tissue disease; 900 control patients and 100 normal blood donors were examined simultaneously. The incidence of anti-Sm antibodies in French SLE patients was low compared with the occurrence observed in similar studies in USA (even when highly sensitive assays were used) but was of the same magnitude as European results. Frequency of anti-Sm antibodies in SLE patients varied moderately when detected by immunodiffusion (12%), or by immunoblotting (17%), however, it seems that the ethnic and/or genetic background of patients induces more significant differences. SLE patients from the French West Indies had anti-Sm antibodies in 39% of cases when detected by immunodiffusion and in 50% when immunoblotting was used. In these patients the incidence of the antibodies was five times more frequent than that of mainland French patients. Immunization against snRNP does not seem to be a common feature of all SLE patients.