Hen egg albumin (HEA), heavily conjugated with dodecanoic acid (D-HEA), stimulated sustained delayed type hypersensitivity (DTH) specific for HEA without detectable antibody formation in guinea pigs. An oil-in-water emulsion containing purified BCG cell walls attached to the oil drops was found to be a very effective adjuvant for enhancing DTH to D-HEA, but not to HEA. Animals immunized with D-HEA in the BCG cell wall emulsion produced skin test reactions 2.4 cm in diameter when challenged with HEA 21 days after a single immunization. Control animals immunized with D-HEA in saline produced skin reactions 1 cm in diameter to similar challenge. Neither group of animals produced detectable antibody to HEA-OR D-HEAmThe emulsion had no adjuvant effect if the BCG cell walls were suspended in the aqueous phase and not attached to the oil droplets. Dodecanoic acid conjugated Salmonella typhi organisms could be used in place of the BCG cell walls to produce effective adjuvant preparations. Fruend's complete adjuvant and other water-in-oil emulsions, however, were found to be ineffective adjuvants for enhancing the degree of DTH produced by D-HEA. Experiments with autoradiography demonstrated that effective adjuvant preparations promote the localization and retention of both 125-I-labeled D-HEA and 125-I-labeled BCG cell walls in the paracortical area of lymph nodes where they are in close proximity to many T-type lymphocytes which proliferate in the induction of DTH.