Group a allotypes not detected by qualitative typing or anticipated from breeding data (latent allotypes) were detected at low levels in 50% of normal rabbit sera tested. The latent allotypes, which were serologically identical to allotypes of pooled IgG, were detected in sera from rabbits with all possible combinations of group a allotypes and their occurrence in individual rabbits was transitory and sporadic. These findings give reason to question the assumption that the synthesis of immunoglobulin allotypes is directed by allelic structural genes.