In a previous study, we have identified endonexin I1 (E-11) on human liver plasma membranes as a specific, Ca2 + -dependent, small hepatitis B surface antigen (HBsAg)-binding protein. In this article, we describe the spontaneous development of anti-HBs antibodies in rabbits immunized with native or recombinant human liver E-I1 and in chickens immunized with the F(ab'), fragment of rabbit anti-human liver E-I1 immunoglobulin G. Anti-HBs activity was not observed in rabbits immunized with rat liver E-11. Cross-reactivity of anti-E-I1 antibodies to HBsAg epitopes was excluded, since anti-HBs and anti-E-I1 activities can be separated by E-I1 affinity chromatography. The existence of an anti-idiotypic antibody is further demonstrated by competitive binding of human liver E-I1 and this antibody (Ab2) to small HBsAg, suggesting that Ab2 mimics a specific E-I1 epitope that interacts with small HBsAg. In addition, it was demonstrated that anti-HBs antibodies developed in rabbits after immunization with intact human liver E-I1 or in chickens after immunization with F(ab'), fragments of rabbit antihuman liver E-I1 immunoglobulin G recognize the same epitopes on small HBsAg. These findings strongly indicate that human liver E-I1 is a very specific small HBsAg-binding protein and support the assumption that human liver E-I1 is the hepatitis B virus receptor protein.