Yeast cells undergo periodic fluctuations in density during the cell division cycle such that a minimum in density occurs at the time of cell separation whereas a maximum occurs between the time of deoxyribonucleic acid replication and nuclear division. Synchronous cultures can be selected from asynchronously growing cell cultures by withdrawing the cells of least or greatest density after banding in Renografin-sucrose density gradients. This technique is rapid, reproducible, and almost unlimited in capacity.