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 γ-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 G1 into S phase. Once the cell is in S phase, these core structures are not needed for the G2-M phase transition.