The study of substrate specificity and activity of proteolytic enzymes secreted by K. pneumoniae strains with different virulence was carried out. The strains were cultivated in a liquid semi-synthetic medium. The biomass was inactivated, and the supernatant fluid was separated from microbial cells by centrifuging. In the supernatant thus obtained and in the fractions isolated by gel filtration with the subsequent purification on DEAE Sepharose elastase-like, trypsin-like and chemotrypsin-like proteolytic activity was determined. In K. pneumoniae strains with different virulence only a single proteolytic enzyme--elastase with a mol. wt. of 21 kD--was detected. The protease activity of the supernatant culture fluid did not depend on the virulence of the strain and was equal to 5,416-7,476 I.U./ml. The activity of the purified enzyme was 100% of the elastase-like activity of the supernatant culture fluid. The most virulent K. pneumoniae strain K2, whose LD50 for white mice was less than 10 microbial cells, was characterized by lower elastase-like activity. The absence of correlation between protease activity and K. pneumoniae virulence may be explained by the fact that surface glycoproteins of eukaryotic cells are glycosilated and thus slightly accessible for proteases.