In experiments with albino rats it was found that after administration of phytobacteriomycin, trichotecin, hygromycin B or levoristatin into the stomach in doses of 1/20 of LD50 activity of the microsomal enzymes of the liver cells significantly changed and the changes persisted within at least 2 weeks. The above antibiotics induced similar changes in the lysosome enzyme, i.e. acid phosphatase, providing an increase in its activity. Changes in the activity of succinate dehydrogenase (mytochondria indicator enzyme), glucose-6-phosphatase (ribosome indicator enzyme) and aspartate aminotransferase (cytoplasm indicator enzyme) were different for each antibiotic. It is concluded that the above antibiotics were capable of impairing on intoxication the enzymatic function of various cell microstructures, though the levels of the change direction may be different.