Casomorphin receptor binding has been demonstrated in opiate receptor assays and bioassays. Duodenal aspirates from human volunteers given cow's milk have demonstrated the presence of several casomorphins, particularly β-casomorphin-7. Few studies have assessed the presence or action of casomorphins in humans. The ability of casein-derived opioid peptides to pass into the bloodstream is not well characterized, and is the subject of the present study.