B-Casomorphin-7 (H-Tyr-Pro-Phe-Pro-Gly-Pro-Ile-OH) and its analogues: B-casomorphin-6, (-5) and (-4) (derived by sequential removal of respectively one, two or three amino acid residues from the C-terminus), were tested for their opioid activities in a variety of assay systems. 

Each of the four peptides displayed opioid activity in an opiate receptor binding assay, the isolated mouse vas deferens (MVD) , the guinea-pig ileum longitudinal muscle myenteric plexus preparation (GPI) and produced naloxone-reversible analgesia after intracerebroventricular injection into rats. In contrast, none of the peptides displayed opioid activity in the isolated rat vas deferens preparation (RVD). B-Casomorphin-5 was the most potent compound in all the assays employed. Each B-casomorphin was more potent on the GPI than on the MVD. In view of the fact that the GPI, MVD and RVD are populated predominantly by p-, 6- and 6-receptors, respectively, the Bcasomorphins probably represent u-type opiate receptor agonists.