DNA-binding proteins made by Chinese hamster ovary (CHO) cells at specific times in their cell cycle were chromatographed on DNA-cellulose and analyzed with sodium dodecyl sulfate polyacrylamide electrophoresis. Some small proteins (less than 25,000 mol wt) were made in S (the period of DNA synthesis), but not during G1 (the period following mitosis and preceding DNA synthesis). These were relatively low in tryptophan and could in part be histones. Larger DNA-binding proteins were made equally in early and late interphase. Proteins from cells labeled in the absence of calf serum during the S period had a different electrophoresis gel pattern compared to cells labeled in the presence of calf serum. Proteins from G1 cells labeled with or without serum had similar patterns. These results suggest that effects of serum are localized temporally in the cell cycle. Comparing our results with those of Salas and Green (Nature (New Biol.), 229, 165 (1971)), we suggest that the G0 state of nongrowing cells is distinct from the normal G1 phase of proliferating cells.