To understand the emergence and evolutionary selection of the efficient mechanisms of innate immunity it is necessary to accumulate knowledge about the structural and functional properties of antimicrobial peptides in different animal species. The cationic antimicrobial peptides, alpha-defensins, were isolated from leukocytic extracts of the lower narrow-nosed monkey, hamadryas baboon Papio hamadryas, using ultrafiltration, preparative electrophoresis and reverse-phase highperformance liquid chromatography. Analysis of the antimicrobial properties of alpha-defensins showed that they display a wide spectrum of antimicrobial activity, comparable with that of human alpha-defensin HNP1, and exert bactericidal and fungicidal effects at micromolar concentrations. A study of the influence of different medium conditions on antimicrobial activity of alpha-defensins revealed that a higher ionic strength or the presence of blood serum leads to a marked decrease in antimicrobial activity of alpha-defensins, while pH has no appreciable effect on it. We found that hamadryas baboon alpha-defensins are able to increase the permeability of the outer and inner membranes of E. coli, suggests that the bacterial membrane is one of the major targets of the antimicrobial effects of these peptides. The revealed differences in antimicrobial activity of alpha-defensins may result from their structural heterogeneity, which reflects different pathways of evolution of alpha-defensins in primates and underlies the selectivity of their antimicrobial effect.