Cell cycle analysis of Chinese hamster ovary (CHO) cells pulsed with 10 μM bromodeoxyuridine (BrUdr) for 30 min and resuspended in fresh medium. Aliquots of the cell suspension were removed every 2 h, fixed and then stained with FITC conjugated goat anti-mouse following primary antibody staining with anti BrUdr antibody. Propidium iodide was used to determine DNA content. Labelled cells are seen to pass out of S-phase at t=0 and into G2/M at 2 h. At 4 h, mitosis has occurred in a proportion of the population leaving some labelled cells in G2/M and others in G0/G1. This is also observed at 6 and 8 h but by 10 h the majority of cells have passed through G2/M into G0/G1 with a small percentage of cells re-entering S-phase.