Mouse chitotriosidase cleaving chitin belongs to the family of mammalian chitinases, whose biological functions are poorly understood. Chitotriosidase activity in mouse serum was shown to be much higher than in humans. The following interstrain differences were revealed in mouse chitotriosidase activity: GR>C57Bl/6>BALB/c>A/Sn>CBA. Chitotriosidase activity in CBA mice was lowest and practically did not differ from that in C3H/He and ICR mice. No sex-related differences were found in enzyme activity. Hybrids of opposite strains CBA and C57Bl/6 were characterized by dominant inheritance of this sign (elevated activity of chitotriosidase in the serum). Intragastric administration of chitin in a single dose of 100 mg/kg was followed by a decrease in chitotriosidase activity in the lungs, but not in the blood serum and homogenate of gastric cells from CBA mice. These data indicate that intragastric administration of chitin does not induce chitotriosidase in mice.