We examined the isotypes of serum amyloid A protein (SAA), a sensitive acute phase reactant in patients with rheumatoid arthritis, rheumatoid arthritis with reactive amyloidosis and those undergoing surgery. The analysis was performed using FPLC system with Mono Q, ion exchange chromatocolumn, for the separation of low molecular fraction, which was obtained by the molecular sieve filtration. Four peaks, I-a, I-b, II and III were separated by FPLC analysis. Peaks II and III were the main isotypes of SAA and I-b was detected only in few cases. Although the recovery was low and varied after the filtration, the results obtained by FPLC were reproducible. Although peak I-a in RA with amyloidosis tended to be less than in other disorders, there was no specific patterns of SAA isotypes corresponding to each disorder.