Chymosin is a gastric aspaitic proteinase dominating m stomachs of many new born mammalians. Chymosin molecules can take up the two alternative structuial forms: the first one has S| and S} binding pockets occluded by its own residue Tyr77 (the selfinhibited form), and the second has these pockets free for a substrate binding (the active form). The preliminary incubation of the enzyme with the pentapeptide corresponding to the histidine-proline cluster of the specific substrate K-casein icsults in 200-fold increase of the hydrolysis rate for the enzyme 'slow substrate'. The iesuli suggests that the cluster plays a role of the allosteric effector promoting the conversion of the enzyme fiom the self-inhibited to the active state. These data provide the explanation of ehymosin specificity towards K-casein.