The antimicrobial activity (the ability to activate the microbial autolytic system) and immunostimulatory activity (the ability to improve the phagocytic cell functioning) of 20 food-protein hydrolysates [five food proteins (casein, alpha-lactalbumin, beta-lactoglobulin, ovalbumin and serum albumin) hydrolyzed with four gastrointestinal proteinases (trypsin, alpha-chymotrypsin, pepsin and pancreatin)] were examined. All the food-protein hydrolysates acted antimicrobially in vitro towards all 24 microbial strains tested: autolysis of 20 naturally autolyzing strains was activated, with the autolysis activation index (K(A)) ranging from 1.04 to 22.0, while autolysis was induced to values of 2.81-56.7% in four naturally nonautolyzing strains. When given to mice per os, all the food-protein hydrolysates enhanced the phagocytosing capacity of peritoneal macrophages, with the enhancement index (K(I)) ranging from 1.02 to 1.41. A direct correlation between K(A) and K(I) was observed. We make the presumption that K(I) is a function of K(A).