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A synthetic peptide consisting of the aminoacid sequence of residues 64-82 of lysozyme, with alanine replacing cysteine as residue 76, was prepared by the solid-phase technique. Mild reduction followed by reoxidation in air of the deprotected peptide led to the formation of a closed loop containing an intrachain disulfide bond. A conjugate consisting of this "loop" attached to multi-poly(DL-alanyl)-poly(L-lysine) elicited, in rabbits and goats, the formation of antibodies capable of reacting with lysozyme and with the loop peptide prepared from it. These immunological interactions can be inhibited by either lysozyme or the loop peptide, but not by the performic acid-oxidized open-chain peptide. Thus, the antibodies elicited by the completely synthetic antigen show specificity toward the "loop" structure (residues 64-80) of native lysozyme.