Colostral IgA from rabbits heterozygous for light chain allotypic specificities controlled by the b locus has been shown by quantitative precipitin techniques to consist of molecules bearing one allelic type or the other but not both. Molecules containing both specificities were obtained by mixing b4 and b5 colostral IgA in 5 M guanidine and dialyzing away the guanidine. These observations indicate that 11 S colostral IgA is synthesized as a dimer in single cells and not derived from 7 S IgA.