Caseinomacropeptide (CMP) is a 64-amino-acid-residue peptide which is released from kappa-casein by gastric proteinases. This review sums up the knowledge concerning its effects on the digestive function. Part 1 recalls the origin and heterogeneity of CMP. Here we underline that there are various forms of CMP which differ by their glycosylation level and genetic mutation. Consequently the forms used for studying biological activities need to be defined accurately. Part 2 summarizes the effects of CMP on digestive secretions. The major effect is an inhibitory effect on acid gastric secretions. Simultaneously, the blood concentration of regulatory digestive peptides is modified. In part 3 we try to clarify the mechanisms of action of CMP. A slightly glycosylated form of CMP, the A variant, appears to be responsible for the biological activity. Evidence suggests that CMP triggers stimuli from intestinal receptors without being absorbed. The signal would be then transmitted to organs through regulatory digestive peptides.