Primary cultures of newborn mouse epidermal cells proliferate rapidly and with a high growth fraction for several months when grown in medium with low calcium (0.02 to 0.1 mM). Addition of calcium to levels generally used in culture medium (1.2 mM) was followed by rapid changes in the pattern of proliferation. By using a combination of technics (a stathmokinetic method, autoradiography, [3H]thymidine incorporation into DNA, DNA flow cytometry) it was found that cell flux was blocked for 5 to 6 h, followed by a short rise in the mitotic rate at 10 h, and a gradual fall in all growth parameters until about 32 h after the calcium switch. There was no accumulation of cells in any particular cell cycle phase. The results indicate that the calcium switch is followed by a strong reduction in cell flux from G1 whereas the majority of the cells that had left G1 at the time of the switch completed one cell division before cessation of all proliferative activity. Both before and after the switch the primary epidermal cultures consisted of one diploid and one tetraploid G1 DNA stemline that seemed to react in the same way to calcium.