"Soluble liver antigen" (SLA) has been reported to be an infrequent but in certain cases a unique marker of autoimmune hepatitis, with cytokeratins 8 and 18 as major antigenic components. Using precharacterized sera, we could confirm trypsin sensitivity and a molecular weight of approximately 50 kD of the reactive protein. However, the reaction differed from that of cytokeratins 8 and 18 by molecular weight and a pI of 7.5. A significant reactivity to cytokeratin 8 and 18 preparations was seen in only 1/12 patients. Immunoscreening of a human liver expression gene bank yielded no clones with sequence homology to cytokeratins. We conclude that reactivity in SLA positive sera is not mainly directed against cytokeratins 8/18 and recommend native antigen preparations for diagnostic use until the exact molecular nature of the 52 kD SLA antigen has been elucidated.