Centrosomes were microsurgically removed from BSC-1 African green monkey kidney cells before the completion of S phase, Karyoplasts (acentrosomal cells) entered and completed mitosis, However, postmitotic karyoplasts arrested before S phase, whereas adjacent control cells divided repeatedly. Postmitotic karyoplasts assembled a microtubule-organizing center containing gamma -tubulin and pericentrin, but did not regenerate centrioles. These observations reveal the existence of an activity associated with core centrosomal structures-distinct from elements of the microtubule-organizing center-that is required for the somatic cell cycle to progress through G(1) into S phase. Once the cell is in S phase, these core structures are not needed for the G(2)-M phase transition.