The Coxsackie virus B1 has been studied for its effect on the chromosome apparatus in cells of mouse feti and newborn mice in case of intraperitoneal infection of females on the 3d week of pregnancy. It is stated that the virus readily moves across the placental barrier and induces chromosome aberrations in the cells of feti, their number being 2.5 times higher than the control level. The maximal number of structural chromosome aberrations was registered during the first three days from the moment of infecting. Then a gradual decrease in the number of aberrant metaphases in the cells of newborn mice was observed. It had reached the value of the control level by the 17th day of observation.