Sera from 107 patients with rheumatoid arthritis (RA), 120 patients with other rheumatic diseases and 60 blood donors were tested with indrect immunofluorescence on the middle third of Wistar rat oesophagus as a substrate for the presence of antikeratin antibodies (AKA). By labelling the stratum corneum and stratum spinosum with IgG antibodies, three patterns of reaction were distinguished. Among them, only one pattern which showed an intense linear laminated fluorescence on stratum corneum and a weak fluorensence on stratum spinosum was valuable for the diagnosis of RA, with a specificity of 99% and a sensitivity of 23%. It was also found that they were not related to the presence of rheumatoid factor or anti-RA 33,000/36,000 antibodies. Thus, AKA might be another marker antibody in RA.