Summary: Quantitative precipitation studies of the antibodies produced in response to the immunization of rabbits immunized with bovine albumin modified by the addition of peptides of glutamic acid, lysine, leucine, or phenylalanine indicated that some of the antibodies were specific for the carrier protein, others were specific for the added polypeptide, and that some require both the modification and at least a part of the carrier protein. Similar quantitative studies on the sera prepared against rabbit albumin which had been modified in the same manner indicated that there were antibodies toward the added polypeptide and possibly some which required a portion of the carrier protein and the modification. The possible explanations for the existence of the antibodies requiring the modification and at least part of the carrier protein are discussed. Absorption experiments on antisera to the modified bovine albumins indicate that similarly modified rabbit albumins precipitate all the antibodies produced in response to the added polypeptide, but that unmodified bovine albumin does not precipitate all of the antibodies and that it may form a nonprecipitating complex with some antibodies which prevents subsequent precipitation with the homologous antigen. The modification of the bovine albumin with various polypeptides was found to weaken the antigenicity of the protein. The length of the added polypeptide chain appears to be more important in the change of specificity than the number of added polypeptide chains.