Аннотация:The levels of chromosomal anomalies in marrow cells of mice kept under conventional conditions in the animal house of the Mechnikov Research Institute for Vaccines and Sera (Moscow) and under specific pathogen-free conditions were compared by the micronuclear and metaphasic methods. In the animals kept under specific pathogen-free conditions the occurrence of anomalies did not exceed the normal level; in the animals of the other group the level of cells with aberrations was 4-5 times higher. All detected aberrations in the chromosomal structure were unstable. The results obtained in this investigation indicate that under the conventional conditions existing in the animal house of a microbiological research institute mutogens of the biological nature may be present.