The injection to mice of a water-soluble form of a chemical allergen (2, 4, 6-trinitrophenylsulfonic acid) in Freund Complete Adjuvant resulted in the development of extremely intensive and persistent (more than 14 weeks) sensibilization which was determined by skin tests. Depending on the challenge, the persistence of skin reactions varied from 48 to 244 hours, but an early component was detected 6 hours after the injection. When this sensibilization method is applied, the effectors of delayed type hypersensitivity are formed in the regional lymph nodes.