To investigate the transplacental cytogenic effect of air pollutants the authors collected samples from Shinchon, Guro, Banpo and Jungnung-dongs in winter season. The air filters were extracted by mixture of benzene and ethanol, then a certain amount of extracted sustance was injected to pregnant mice at 16th day of gestation. From the fetal liver emulsion polychromatic erythrocytes were collected and stained with Giemsa solution. The cytogenic effect was evaluated by micronucleus test by which numbers of polychromatic erythrocytes containing microunclei (MNPCE) per 1, 000 polychromatic erythrocytes could be counted.