We used the autoradiographic method according to Quastler and Sherman to analyse alterations in the cell cycle under the influence of city smog extracts. Investigations were performed on logarithmically growing cultures of kidney and embryonic cells of the Syrian golden hamster. Low concentrations of city smog extracts (0.125 micrograms/ml Benzo(a)pyren-equivalent) induced a remarkable delay of cell entrance into DNA-synthesis. Furthermore a considerable prolongation of generation time and phase of DNA-synthesis was detected. The number of mitosis was strongly reduced. Already a doubling of concentration of city smog extract caused an almost complete breakdown of the cell cycle and a disappearing of mitosis for a time period of 10 hours. Our results strongly indicate that city smog extracts lead to a severe alteration of the molecular biology of the cell. Taking this in consideration, it can be assumed that a long term exposure of human beings to the city smog could induce an injury of health.