The amino acid sequence of the constant (C) domain of the light chain of the mouse myeloma protein M315 has not been identified so far in any other myeloma protein. In this study, serological analysis with antiserum to the C-domain of this light chain (L315) showed that approximately equal to 1% of Igs in normal mouse serum have L chains of the L315 type (called lambda2). Corroborative evidence was obtained by analysis of the carboxyterminal amino acid removed from normal light chains by carboxypeptidase A. A survey of 35 inbred mouse strains showed that all had lambda2; the serum level of Igs with lambda2-chains ranged from approximately equal to 140 microgram/ml in AL/N mice to approximately equal to 25 microgram/ml in SJL, BSVS, and eight other strains. In accord with the anti-Dnp activity of M315, sera from mice immunized with Dnp-KLH had three- to fivefold more lambda2 than sera from control mice immunized with KLH. It was also possible to measure serum immunoglobulin molecules bearing the lambda2 variable region of M315 (VL315). In BALB/c sera, the concentration of VL315 was about sixfold lower than that measured for lambda2. Thus, lambda2-chains are divided into at least two subsets: those whose V domain is indistinguishable from VL315 and those whose VL differs from VL315. A 10-fold increase in VL315 was obtained by immunizing BALB/c mice with Dnp-KLH. The relationship of the VL domains of normal immunoglobulin lambda2-chains to the embryonic Vlambda gene recently sequenced by Tonegawa et al., is discussed.