We measured pyruvate oxidation in intact leukocytes and fibroblasts by measuring 14C02 production. The optimal pyruvate concentration appeared to be higher than that usually applied. Activities remained constant during the incubation and were proportional to the amount of tissue protein added. Man values (+/-SD) were 2.8 +/- 0.9 nmol/h per 10(6) cells and 37 +/- 14 nmol/h per mg of protein for leukocytes and fibroblasts, respectively, for 1-14Cpyruvate oxidation; and 2.1 +/- 0.8 nmol/h per 10(6) cells and 18 +/- 7 nmol/h per mg of protein, respectively for 2-14Cpyruvate oxidation. We compared oxidation rates of pyruvate and 2-oxoglutarate by intact cells with those of isolated mitochondria. The ratio of 14CO2 production vs. activity of mitochondrial marker enzyme demonstrated that the rate of pyruvate oxidation can adequately be assayed in intact cells, but that the permeability of the cell membrane is rate-limiting in the oxidation of 2-oxoglutarate. No significant oxidation of other intermediates of the citric acid cycle was found, presumably owing to a low rate of transport of these substances across the cell membrane.