The regulation of cell cycle progression in normal mammalian cells is dependent on the presence of growth factors. In their absence, non-transformed cells will stop dividing and enter the quiescent state (G0). We show here that in Chinese hamster ovary cells, at least two serum-dependent points exist during G1 that lead to different cellular responses. The first point is located immediately after mitosis and is suggested to link with apoptosis. The second point is located late in G1, and probably corresponds with the 'classic' restriction point R. Cells depleted of serum after the first restriction point will not stop randomly in G1 but continue G1 progression until they reach the late restriction point, as marked by translocation of p42(MAPkinase) (ERK2) to the nucleus.