In an effort to better define the antibody repertoire to streptococcal group A carbohydrate (GAC), somatic cell hybrids were prepared from A/J mice immunized with streptococcal vaccine. Most antibodies were IgG3K and IgMK, while 2 of 26 antibodies were λ type. Each of the IgG3 antibodies had a distinct isoelectric point consistent with previous estimates of clonal repertoires of ~200. IEF analysis of the L chain, however, showed that about half of the antibodies produce a common L chain, called VK1(GAC), previously identified in A/J anti-GAC serum antibodies. Additional support for the structural similarity of these L chains was gained by developing an idiotype antiserum to VK1(GAC). All proteins with the common L chain spectrotype react strongly with anti-VK1(GAC). Thus, it appears that anti-GAC antibodies are composed of H chains bearing a few V(H) regions pairing with a few L chains.