The aim of the work was to study the effect of HCBS, LCBS and LCBSP on the frequency of dominant lethal mutations (DLM) in mice under the genotoxic effect of the experimental mutagen - cyclophosphamide (CP). Eleven groups of outbred mice were formed for the work, 6 males and 36 females in each group. HCBS, LCBS and LCBSP were administered intramuscularly to males once at a dose of 0.5 ml/kg separately or together with intraperitoneal administration of CP at a dose of 60 mg/kg. The studied pharmaceutical substances were also administered three times with an interval of 24 hours before the administration of CP in doses similar to a single administration. To study DLM, female mice were implanted with males three times with an interval of one week. On the 15-17th day after implantation with males, pregnant females were removed from the experiment, autopsied and the frequency of postimplantation mortality (PIM) of embryos was determined, which correlates with damage to spermatozoa, mature and immature spermatids in males. The value of PIM in the experimental groups was compared with that of the negative and positive control groups. Single and course administration of HCBS to male mice with CPinduced cytogenetic instability resulted in decreased clastogenic damage to spermatozoa and late spermatids by CP. Course administration of LCBS and LCBSP to male mice before CP injection induced a significant decrease in postimplantation mortality of mouse embryos, indicating a decrease in the mutagenic effect of CP on mature spermatozoa. Thus, HCBS, LCBS, and LCBSP exerted an antimutagenic effect on germ cells of male mice under conditions of CPinduced cytogenetic instability.