Two antisera detecting different allotypic specificities, A1 and B1, of cattle immunoglobulins were produced by isoimmunization with purified IgG in Freund's complete adjuvant. The A1 marker was found to be restricted to the Fc portion of IgG2 heavy chains, whereas B1 was a marker of immunoglobulin light chains and was found on IgG, IgA and IgM molecules. Genetic analysis indicated that the alleles controlling A1 and B1, bva1 and bvb1, respectively, were autosomal dominants and that they were inherited independently. The bva1 allele was highly frequent in 11 breeds except Hereford, which showed a low frequency. In contrast, bvb1 was infrequent or absent in all breeds.