Antisera against rat liver aspartate aminotransferase (EC 2.6.1.1) isozymes were used to study the activity and immunologic pattern of these isozymes in the livers of the rat, mouse, hamster, gerbil and in Ehrlich ascites cells. A double immunodiffusion precipitin test and immunoelectrophoresis showed that, except for the gerbil, there was a pattern of identity of AAT isozymes in the presence of either the antianionic or the anticationic antisera. Although gerbil AAT isozymes are immunochemically different from those of the other rodents studied, they were inactivated by the respective antiserum in a manner similar to that observed with the other species. This may suggest that antigenic determinants at the catalytic site of each of the liver aspartate aminotransferase isozymes are least likely to change throughout the evolutionary process.