The complete sequence of rabbit antibody light-chain L 120 has been elucidated. The antibody was raised against Micrococcus lysodeikticus bacteria and is specific for the external part of the cell wall. All protein used in this work was obtained from a single 50-mL bleeding. The variable region of L 120 is compared to 13 other sequences of chains of different specificities. The constant region of this b4 k chain is identical to that of two other constant regions published earlier. The general structure of the rabbit light chain is compatible with the three-dimensional folding proposed for human myeloma chains.