Microcells, cytoplasmic fragments that contain micronuclei composed of one or a few chromosomes, can be generated directly from mitotic cells. Cytochalasin B, which causes nuclear extrusion in interphase cells, has a similar effect on the chromosomes of colcemid-blocked mitotic cells. The forces generated during centrifugation in a Percoll gradient are sufficient to separate the extruded microcells from the parent cell. The chromosomes contained in an extruded microcell form micronuclei during the process, and in all respects are comparable to microcells generated from micronucleated cells except that they are uniformly in the G1 phase of the cell replication cycle. The procedure is probably applicable to all mammalian cells that grow in culture and can be employed to make microcells for the transfer of both intact and fragmented chromosomes.