The contribution of glycolysis and Krebs cycle to the energy provision of human blood lymphocytes in the presence of phytohaemagglutinin has been investigated. Harvesting cultured cells at different times and reincubating them allowed measurement of the potency of metabolic processes at any moment. It was found that the increase in several parameters of carbohydrate metabolism as seen with PHA during 4 h, is more pronounced after 1 or 3 days. During this stimulation oxidation of substrates in mitochondria or nuclei remains tightly coupled to phosphorylation of ADP. It was calculated that under normal conditions 15% of the ATP is formed in glycolysis, and 85% in oxidative phosphorylation. Either of these pathways can provide the energy for the PHA stimulation in short-term experiments. However, both pathways must be active for long-term incubations.