To obtain information on anti-RA 33 as a marker antibody of rheumatoid arthritis (RA), a panel of Finnish sera were tested by immunoblotting with partially purified RA 33 antigen. Six of 100 specimens from patients with RA, one of 39 specimens from subjects who later developed seropositive RA, and one of 50 specimens from subjects with "false-positive" rheumatoid factor reactions were positive. The findings are compatible with the concept that anti-RA 33 is associated with RA from its onset and can even precede the disease. However, the prevalence of this antibody in Finnish RA patients is remarkably low compared to patients from continental Europe.