A small proportion of the antibodies to Group A streptococcal carbohydrate (A-CHO) elicited in BALB/c mice by immunization with Group A streptococci, has idiotypic determinants in common with the BALB/c myeloma protein S117 which has specificity for N-acetyl-glucosamine, the major antigenic determinant of A-CHO. The expression of these idiotypic determinants is under the control of a gene which is linked to the Ig-1a+ allotype locus in strain BALB/c and in other strains carrying the same Ig-1 haplotype. This gene (S117+) segregates in breeding experiments as if it were an allele to the gene A5A+ which controls the expression of the A5A idiotype in association with antibodies to A-CHO in strain A/J and which is linked to the Ig-1e allotype locus. Another possible allele, linked to the Ig-1c allotype locus, controls the expression of both S117 and A5A cross-reactive determinants (S117cr, A5Acr). The distribution of these idiotypic determinants in various lines that carry recombinant Ig-1 haplotypes suggests that the A5A and S117 loci are nonallelic and map at different positions in the Ig-1 region. The data suggest complex pseudollelic relationships between different Ig-1 haplotypes that allow the expression of the same genes in allelic and in nonallelic fashion.