The effect of antibodies to the light chain variable region (VL) of protein MOPC‐315 (a, Λ2), on the binding of hapten by VL315 dimer or Fv315 (VL + VH) was studied by equilibrium dialysis. Anti‐VL did not change the binding properties of Fv but affected the binding properties of VL dimer. At pH 5, the binding properties of VL in the presence or absence of anti‐VL were the same, whereas at pH 8, anti‐VL reduced the number of ligands bound to VL from two to one. It has previously been shown that VLdimer binds one ligand at pH 5 and two ligands at pH 8, and that VL conformation at pH 5 is tighter. Hence, our results suggest that anti‐VL tightens the conformation of VL dimer at pH 8.0 such that it can bind only one ligend. Since Fv is not affected by anti‐VL, the results indicate that a combining site made of two identical chains (VLdimer) can undergo a conformational change upon interaction with its antibody. Such conformational change can indirectly affect the binding properties.