The amino acid sequence of a wild rabbit light chain constant region of allotype b95 was nearly completely determined by manual and automated Edman degradation procedures. The comparison of the b95 primary structure with the other b allotypes reveals about 20% substitutions between b95 and b4, b5 and b6, and 36% between b95 and b9. The substitutions are clustered in parts of the chain in agreement with our sequence data for b5 and b6. The presence in b95 of the characteristic cysteine residue at position 170 and the tryptophane residue at position 147 is in agreement with the serological similarity of these various rabbit kappa light chains. The examination of the rate of amino acid substitutions between the b95 chains and the other b allotypes shows that b95 is closer to b4, b5 and b6 than to b9.