The antibody response to DNP-hapten in inbred mice injected with different carrier proteins was studied under controlled conditions. Use of isogenic serum resulted in the production of antibodies to DNP but none toward the carrier protein. Immunization with DNP conjugates of bovine, rabbit, and rat 2 globulins resulted in anti-DNP titers which were 13, 3 and 3 times as high, respectively, as with the isologous DNP-2 globulin conjugate. The responses against carriers paralleled the anti-DNP responses (bovine > rabbit = rat > mouse = 0). The degree of anti-hapten response was directly related to the antigenicity of the carrier molecule. Due to anti-hapten production involving recognition of the "foreignness" of the carrier or modified carrier.