Antisera were raised in rabbits against L chains, isolated from mouse myeloma protein MOPC21 (gamma 1, chi V chi 15 group). Specific antibodies for the V and C domain of MOPC21 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 chin-monomers and dimers isolated from the L chain preparation was compared to that of intact IgG1 using binding inhibition of 125I-labeled IgG1 on the antibody-containing immunosorbents. It was established that changes of IgG1 quaternary 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 MOPC21 V1-like normal IgG in mouse serum of 6 inbred strains using antibodies against the V1 domain. Data obtained by inhibition of radioimmunoadsorption, indicate that in C57BL/6 mice 0.08% of normal serum Ig carries a V1 region which is idiotypically related to the V1 of MOPC21. In serum Ig of BALB/c mice the percentage is 0.16.