CNBr cleavage of rabbit heavy (H) chains leads to the formation of a fragment, C-1, which consists of the N-terminal half of the H chain. Fragment C-1 is cleaved at methionyl residues but held together by intrachain S-S bonds so that smaller fragments can be liberated by total reduction and alkylation. In the case of the C-1 fragment from an anti-p-azobenzoate antibody preparation, which has a light (L) chain of markedly restricted heterogeneity, total reduction and alkylation liberated seven major fragments in good yield. The N-terminus of two of these fragments corresponds to position 35 of the H chain but their N-terminal sequences are clearly different. The H chain regions represented by the other fragments implied that they were derived from H chains having different distributions of methionyl residues. This hypothesis was supported by isolating six different antibody components from the antibody preparation by isoelectric focusing and then digesting them with CNBr. Comparison of the products showed that the six components all appeared to behave differently. These results are interpreted as suggesting that the process whereby H and L chains are paired in vivo may not be completely specific and may provide a simple means of generating a significant contribution to antibody diversity.