Chinese hamster cell cultures were treated with different concentrations of lead acetate for a period of one cell cycle. Afterwards chromosome damage and disturbance of the mitotic apparatus were investigated. With increasing lead acetate concentration the mitotic index decreased. However, structural chromosome aberrations, which can be related to true discontinuities of the chromosome structure, and disturbance of the chromosome distribution were never more frequent than in controls. On the other hand, the achromatic lesions of gaps increased with the most concentrated lead acetate solution.