This is the first report of the elimination of confronting cisternae (CC), prominent organelles in dividing HeLa cells, upon experimental manipulation of the cells. CC are lost when a double thymidine block is employed to synchronize cell division. This observation is consistent with the hypothesis that the occurrence of CC in some fetal cells and in selected tumor cells depends on the speed at which the cells cycle through mitosis. If thymidine blocks DNA synthesis and arrests cells at the G1/S interface, then thymidine probably has an indirect effect on CC. This paper reports the effect of thymidine on the occurrence of CC and briefly discusses how inhibitors of membrane synthesis or microtubule polymerization may affect the occurrence of CC during mitosis.