Calf chymosin molecules exist in the two alternative structural forms: the first one has S1and S3binding pockets occluded by its own Tyr77residue (the self-inhibited form); the second has these pockets free for a substrate binding (the active form). The preliminary incubation of the enzyme with a pentapeptide corresponding to the histidine - proline cluster of the specific substrate κ-casein results in a 200-fold increase of the hydrolysis rate for the enzyme 'slow substrate'. The result suggests that the cluster is an allosteric effector that promotes the conversion of the enzyme into the active form. These data provide the experimental ground for the explanation of chymosin specificity towards κ-casein.