The amino acid sequence of a rabbit immunoglobulin light chain of allotype b5 has been nearly completed. A comparison of its structure with that of light chains of allotypes b4, b6, and b9 confirms that the constant regions of these various kappa chains differ by 20-35%. The substitutions are clustered in parts of the second half of the chain, and the b5 form bears more resemblance to the b6 chain than to any other, in good agreement with previous serological data. The analysis of the variable region reveals the existence of certain allotype-associated residues which have also been reported in other b5 chains, but not in proteins of the other allotypes. An examination of the rabbit light chain sequences between positions 96 and 107 suggests that this portion of the chain may be encoded separately by a joining "J" DNA segment, as has been described previously for murine and human immunoglobulins. In the rabbit, however, these J kappa regions appear to differ from one allotype to another. Together with the extensive variations of the constant regions, these data suggest that the rabbit kappa gene organization more closely resembles the murine gamma system (four different C gamma genes each flanked by its J segment) than the murine kappa system (a single C kappa gene).