Latent group a allotypes were detected with a sensitive radioimmune inhibition assay. Sera, IgG preparations, and antibody fractions containing these allotypes inhibited the binding of insolubilized allotypic antisera to various radiolabeled antigens including IgG pools, homogeneous antibodies, and, in the case of a3, a VH fragment from a3/b4 IgG. Several different group a antiallotypic sera were used in the assays and all gave similar results. Comparison of inhibition curves for nominal and latent allotypes indicated that the full spectrum of allotypic subspecificities may be expressed in latent allotypes. Hemagglutination studies carried out with five sera containing high levels of latent allotypes confirmed the results obtained with the radioimmunoassay and indicated that inhibition values did not, at least in four of the five samples studied, reflect the presence of antiallotype antibodies.