Three methods of cell cycle analysis, involving the use of tritiated thymidine, have been applied to asynchronously dividing suspension cultures of sycamore. Conditions for an effective chase of unlabelled thymidine were established from a study of the kinetics of entry and incorporation of tritiated thymidine into the cells. The levels of thymidine used did not affect the rate of cell division or the duration of the phases of the cell cycle. The analyses of the cell cycle based upon pulse labelling, continuous labelling, and a combination of densitometry and autoradiography were in good agreement and showed that the phases S (mean 7.0 h), G2 (mean 8.5 h) and mitosis (mean 3.0 h), were of relatively constant duration, whereas G1 was of variable duration. No relation between nuclear DNA content and mitotic-cycle time or the duration of S-phase could be inferred from the data presented.