Opioid antagonists were sought in the fragments of κ-casein which were obtained by chemical synthesis and enzymic digestion. A synthetic bovine κ-casein peptide (35–41), Tyr-Pro-Ser-Tyr-Gly-Leu-Asn (casoxin A) showed opioid antagonist activity at 200 μM in the guinea pig ileum assay. A synthetic peptide Tyr-Pro-Tyr-Tyr (casoxin B) which is found in bovine and human κ-casein, also showed opioid antagonist activity at 100 μM. Another opioid antagonist peptide (casoxin C) was isolated from tryptic digests of bovine κ-casein by reverse-phase HPLC. The structure of the peptide was Tyr-Ile-Pro-Ile-Gln-Tyr-Val-Leu-Ser-Arg, which corresponded to κ-casein (25–34). Casoxin C was active at 5 μM in the guinea pig ileum assay. Thus, bovine κ-casein contains three potential opioid antagonist sequences.