Immune responses to conserved, immunogenic homologues of the mycobacterial 65 kDa stress protein (SP65) have been implicated in inflammatory arthritis. Serum anti-SP65 was measured in AS, RA and healthy controls using an indirect enzyme immunoassay with recombinant SP65. IgA anti-SP65 was elevated in 19 of 59 AS patients, but the elevation in median level was not statistically significant. Anti-SP65 of all isotypes was increased in RA, but achieved significance (P less than 0.01) for IgA only. Adjusting specific antibody results for elevations in total serum Ig levels reduced AS and RA anti-SP65 to near normal levels, suggesting that a major component of the increased anti-SP65 may be secondary to polyclonal activation.