The calcium requirements during mitogenic stimulation of quiescent 3T3-cells in low or in high serum concentration was investigated. It was found that calcium played an equally important role for growth stimulation in low as in high serum concentration. Calcium was not required during the first 6 hours after mitogenic stimulation. However, calcium had to be present thereafter for the cells to initiate DNA-synthesis. The absence of calcium during the first six hours after onset of stimulation did not delay the initiation of DNA-synthesis.