Analysis of A/J antibody to phosphorylcholine (PC) revealed a striking degree of similarity to PC-binding myeloma proteins of BALB/c origin. By quantitative idiotypic analysis A/J anti-PC antibody was composed to antibodies bearing binding site idiotypic determinants indistinguishable from two different BALB/c myeloma proteins, T15 and M511. Idiotypic determinants of three other PC-binding proteins, W3207, M167, and M603 were not detected. Isoelectric focusing of the light chains verified the presence of antibodies similar to T15 and M511 and indicated the presence of a third antibody whose light chains had a pI identical to that of M603. When the sequence of A/J heavy chains were compared to the heavy chains of T15, M511, and M603, both the framework and first complementarity regions were identical in all cases. Sequences analysis of the light chains through part of the first complementarity region revealed three chains, one similar to each of the myeloma proteins T15, M603, and M167-M511. The latter two sequences differ by only a single amino acid (a single base substitution) in the first 23 residues, suggesting that the two light chains may be very similar if not identical. Thus, BALB/c and A/J mice which differ genetically at multiple loci including the heavy chain allotype complex locus show a remarkable preservation of their anti-PC antibodies. These results indicate that the genes encoding these antibodies are contained in the germ line.