Аннотация:The ability of two inactivated bacterial vaccines, from Proteus vulgaris and Klebsiella pneumoniae, to injure synaptonemal complexes (SCs) was studied by means of electron microscopy. The preparations were given intraperitoneally to C57BL/6J male mice during five successive days and testes were fixed 24 h after the last injection. Cyclophosphamide was used for positive control. The vaccine from Klebsiella given at subtoxic dose, which was about 200 times higher that that given to people during vaccinations, induced the 10-fold rise in the frequency of SC abnormalities in murine 1-storger spermatocytes. Breaks of SCs and of single lateral elements of SCs predominated over other types of vaccine-induced anomalies. According to the preliminary data, vaccine from Proteus at subtoxic dose showed no damaging SC activity. The results of the given study are discussed in connection with the negative data obtained earlier when genotoxicity of these two vaccines had been studied in the Ames test, in routine investigations of bone marrow metaphases of vaccinated mice as well as under light microscopy of their SCs.