SUMMARYSertim IgG, labelling ihe straiutn corneum ofthe rat oesophagus epithelium, so-called anii-keratin antibodies (AKA) conslilule the most speciiic marker for ihe diagnosis of rheumatoid arthritis. In this study, we investigated 31 IgG AKA-positive rheumatoid sera and 21 conlrol sera from patients wilh non-rhcuniatoid intlammatory rheutnatic discuses. The serum level of IgG 1.2.3 and 4 was determined by radial inimunodiffusion and the subelass distribtition of IgG AKA by a three-step semi-quantitative immunofluorescence assay using standard monoclonal antibodies specific for each of ihe four human IgG stibelasses. In the rheumatoid sera, the serum level of IgG I was found to be signifieantiv increased and the level of lgG2 signitkaTitly decreased with regard to the control sera. while the levels of lgG3 and 4 as well its lolal IgG were in the nortnal range. lgGI.2..V and 4 AKA were detected in 27(87'Ki).6( 19'^,), 4(13''")iind 11 (35"'ii)ofthe3l rheumatoid sera, respectively, and were found to be independent of Ihe clinical and biological indices of ihe disease. In spile of inlerindividtial heterogeneity, two predominant profiles were distinguished: IgGl(alone) and lgG( I +4). which together represented 18 sera (58"n). The large predominanee of IgGI AKA and the qtiasiabsence of lgG2 AKA suggest thai the recognized anligen may be partly comprised of protein.Moreover, the high frequency of occurrence of !gG4 AKA might result from chronic exposure to the eliciting antigen, which eould be a genuine atitoantigen since we demonstrated that it is also present in ihe stratum corneum of human epidermis.