A technique for the induction of sensitization in pigs with a nickel-alanine conjugate has been reported by other investigators. Similar results were observed in our experiments in mini-pigs. However, using the identical technique, we were unable to induce sensitization in guinea pigs with NiSO4, with nickel-alanine or other nickel-amino acid complexes, or with a complex of nickel with soluble guinea pig skin extract. These results indicate that nickel-amino acid complexes and nickel-guinea pig skin complex were not antigenic in guinea pigs with this technique. Hypotheses for the apparent failure to induce sensitization are proposed.