Centrosomcs of rat kangaroo kidney cells (PtKi) were irradiated with an argon laser microbeam at different stages of mitosis. Microirradiation of one of the centrosomes at prophase resulted in a delay of cells at metaphase. Then the irradiated cells passed directly to interphase. The same microirradiation at the early prometaphase caused irregular cytokinesis without chromosome separation. Microirradiation at late prometaphase and at metaphase did not affect the later stages of mitosis. The electron microscopic studie did not reveal any differences between the irradiated and control centrosomes The data obtained show that centrosomes are not only the microtubule-organizing centre but they also control the processes of cytokinesis, reorganization of chromosomes, etc.