Bacitracin A and its dinitrophenylated derivatives were investigated for antigenicity because they offer several unusual characteristics: a) they are small molecules composed of twelve amino acids; b) they have well-defined structures; c) the dinitrophenyl hapten groups couple to the bacitracin A molecule at known positions and d) bacitracin and its dinitrophenylated derivatives may be obtained in a highly purified state. These properties make bacitracin A and its derivatives suitable for use in the investigation of the nature and characteristics of antigenicity and the homogeneity of the antibody population obtained after immunization with a relatively simple antigen. Materials and Methods. Animals. Hartley strain albino guinea pigs weighing about 400 g, obtained from the same dealer but not from an inbred colony, were used for immunization. Antigens. Bacitracin A (Fig. 1) (1) was coupled to 2,4-dinitrophenyl groups according to the method of Craig, Konigsberg and Hill (2), and the products were purified by countercurrent distribution.