Latent group b markers were detected in sera, in IgG preparations, and on isolated L chains from rabbits bred for homozygosity at the b locus. Serologic analysis of sera from an extended family of homozygous b4 rabbits revealed the presence of latent b allotypes in 5 of 37 sera tested. Latent b5 and b9 markers were identified; none of the sera tested contained latent b6. In two instances, the level of latent b9 allotypes was sufficiently high to permit isolation and detailed serologic characterization of the immunoglobulin population bearing this allotype. The fact that latent allotypes were detected in pedigreed homozygous rabbits minimizes the possibility that lymphoid cell chimerism is involved in latent allotype expression. Furthermore, characterization of the b9 IgG population indicates that the latent allotypic determinants do not reside on a subset of molecules with dual allotypic reactivity.