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SummaryThree different established strains of mammalian cells were grown in chemically defined medium in large cultures. The degree of proliferation of cells of an established strain from human skin in large stationary cultures was significantly greater in the presence of methylcellulose (medium NCTC 135M) than in its absence (medium NCTC 135). The relatively fragile cells of a derivative of monkey kidney LLCMK2 strain were carried in large stationary cultures through 11 transfer generations during 152 days. The presence of methylcellulose was associated with higher cell population levels, proliferation rates, and cell viability. Cells of this strain utilized glucose a t an extremely high rate; during two representative periods the rate averaged 1.2 mg/106 cells/day in cultures on medium 135M and 1.9 mg in medium 135.In a 53-day experiment with mouse fibroblast 2071-L cells, the cells in suspension cultures during the first 28 days went through the normal lag, logarithmic plateau, and initial decline phases in medium 135M, and then were transferred t o large stationary cultures, where they proliferated for 7 days at uniformly high rates in both medium 135 and medium 135M. It appeared that cells of strain 2071-L in such stationary cultures had no need for Methocel as a protective agent. Glucose utilization rates while these cells were carried in large stationary cultures averaged 2 4 times the rates while they were in suspension cultures: about 0.8 and 0.2 mg/106 cells/day, respectively.