Recent data concerning the expression of latent allotypes (allotypes present in low concentration and not anticipated from breeding data) are presented. Using various sensitive assays, latent allotypes of groups a, b and d have been observed in sera, IgG samples, isolated antibody fractions and on lymphoid cell surfaces. Recently, molecules bearing latent group a allotypes have been isolated from IgG samples and from specific antibody fractions by immunoadsorbent chromatography and subsequently typed for VH and CH allotypes. In several preparations latent group d allotypes were observed. IgG clearance rates were measured by intravenous injection of rabbits with differentially radiolabelled (125I and 131I) allotype matched and non-matched IgG samples. In no instance was allotype matched IgG cleared faster than non-matched, although the converse was true in several rabbits, suggesting in vivo recognition of allotypes as a possible regulatory mechanism. Genetic models that can account for latent allotypes require the presence of information in the genome that is not expressed under normal conditions, and, furthermore, these models must include inherited regulation mechanisms to govern synthesis from this information. Available data do not require that all Ig genes are present in all members of a species, but may suggest the presence of genes for limited groups of allotypes.