AbstractAntisera were raised in rabbits against L chains, isolated from mouse myeloma protein MOPC21 (γ1,x, Vx15group). Specific antibodies for the V and C domain of MOPC 21 L chain were obtained by cross‐immunoadsorption of the antisera. The pure anti‐V and anti‐C antibodies were fixed on diazocellulose and used as immunosorbents. The inhibitory capacity of L chain‐monomers and dimers isolated from the L chain preparation was compared to that of intact IgG1using binding inhibition of125I‐labeled IgG1on the antibody‐containing immunosorbents. It was established that changes of IgG1quaternary structure influences the conformational state of the L chain V domain only. The inhibitory capacity of the V domain is 1000‐fold lower in L monomers, if compared with native IgG1, and only 10‐fold lower than in L dimers. The inhibiting capacity of the C domain, however, does not differ in L monomers and intact IgG1. Thus the conformational rigidity of the C domain co‐exists with conformational flexibility of the V domain on the same polypeptide chain. We tried to estimate the content of MOPC 21 VL‐like normal IgG in mouse serum of 6 inbred strains using antibodies against the VLdomain. Data obtained by inhibition of radioimmunoadsorption, indicate that in C57BL/6 mice 0.08% of normal serum Ig carries a VLregion which is idiotypically related to the VLof MOPC 21. In serum Ig of BALB/c mice th