An investigation of the isotopic exchange (turnover) of carbon among the basic groups of organic substances of yeast biomass (proteins, polysaccharides, lipids, free amino acids, carbohydrates, and organic aicds) showed that in the presence of a carbon source and all the necessary nutrient elements in the logarithmic phase of growth of the culture, the bulk of the organic substances of the cell are extremely stable. Under these conditions, no appreciable utilization of the carbon of low-molecular-weight pools, nor any significant degradation of high-molecular-weight substances can be detected. Nor was there any redistribution of carbon-14 among individual components of the fraction of free amino acids and lipids. It was shown that the fraction of exchangeable carbon of the biopolymers of mature cells, like the fraction of the exchnage pool of low-molecular-weight substances, does not exceed 10% of the total carb-n of these substances. It is suggested that there is no mixing of the old and newly synthesized low-molecular-weight carbon pools in the parent cell, and all the components of the biomass of the daughter cells are synthesized chiefly from the carbon of the substrate.