Besides rheumatoid factor (RF), antikeratin antibodies (AKA) and antiperinuclear factor (APN), anti-RA 33 antibody has been described as a highly specific antinuclear antibody for rheumatoid arthritis (RA). In this study RA 33 antibodies were detected using Western blotting with HeLa cell nuclear extract in a group of 94 RA patients and 259 controls. Anti-RA 33 was present in 35% of 94 RA patients, with a similar frequency in RF positive (32%) and RF-negative (45%) RA patients. RA-33 antibody was also present in 60% of a group of 30 patients with anti-U1 RNP positive mixed connective tissue disease. The specificity of anti-RA 33 for RA was 84.6%. Anti-RA 33 antibody was already present in sera from 23.5% of 18 patients with RA of less than one year's duration. Anti-RA 33 antibody was the only positive immunological marker in 3/20 cases of seronegative adult RA. No correlations were found between anti-RA 33 antibody and AKA or APF. Patients with erosive RA and patients whose ESR was > or = 50 mm after 1 hour were more likely to be anti-RA 33 positive (47.6% vs 24.4% and 42.8% vs 29.4%). These results suggest that anti-RA 33 antibody, in the absence of anti-U1-RNP antibodies, can be added to the list of the helpful serological markers for rheumatoid arthritis.