Additional evidence has been obtained to show that different guinea pig anti-hapten antibodies differ in the structure of their L polypeptide chains. Antibodies from animals immunized with the same hapten conjugated to different carrier proteins gave similar starch gel electrophoretic patterns after dissociation of their chains. In a study of fine differences of specificity, cross-reacting antibodies were found to have some L chains with the same electrophoretic mobility. The multiplicity of L chain bands found in the characteristic starch gel electrophoretic patterns of dissociated anti-DNP antibodies was shown to be a reflection of the heterogeneity of antibodies of slightly different specificities. Reduction and alkylation of the active fragment produced by digestion of antibodies with papain yielded starch gel electrophoretic bands corresponding in mobility to L chains. The results are consistent with the notion that L chains are involved in the acquisition of immunologic specificity.