Sm antigen from rabbit thymus acetone powder was purified using a combination of ammonium sulphate precipitation, DEAE-Sephacel and hydroxyapatite chromatography. This preparation was devoid of previously identified nuclear antigens including ribonucleoprotein (U1-RNP), proliferating cell nuclear antigen (PCNA), Sjögren's syndrome antigen A (SS-A/Ro), Sjögren's syndrome antigen B (SS-B/La), Sjögren's lupus antigen (SL), scleroderma antigen 70 (Scl-70), DNA and histones. The purified material was used in an enzyme linked immunosorbent assay (ELISA) to detect anti-Sm antibody. All sera with precipitating Sm antibody detected by immunodiffusion gave reactions in ELISA greater than 0.40 OD405 and contained predominantly IgG anti-Sm antibody. Of 112 sera which did not have anti-Sm by immunodiffusion there were five which gave reactions greater than 0.40 OD405. Four of these five sera contained only IgM antibody and the fifth contained both IgM and IgG. Of these five, one came from a 'normal' control who had a positive anti-nuclear antibody (ANA), facial rash and diabetes, two were from patients with systemic lupus erythematosus (SLE) and two were from patients with mixed connective tissue disease (MCTD). These findings demonstrate that there are patients whose anti-Sm response may be restricted to IgM and in some of these patients the clinical presentation may be different from that of classical SLE.