Structural analysis of immunoglobulin light chains has been carried out in an attempt to elucidate the genetic mechanisms involved in antibody synthesis. Analysis of two mouse kappa-chain proteins is almost complete. The differences are localized in one-half of the molecules, and do not reflect the operation of any one mutational mechanism. The peculiar character of the differences is discussed with reference to various theories of antibody formation. The finding that the two proteins differ in size is incompatible with certain proposed theories.