A total of 40 skin fibroblast cultures from pediatric cases of lactic acidosis were subjected to a series of tests designed to elucidate the nature of an underlying defect in metabolism. Of these 40 cases, in 14 we were able to define the following problems. Pyruvate carboxylase deficiency was evident in five cases showing < 10% normal activity. Phosphoenolpyruvate carboxykinase deficiency was evident in one case where the whole cells showed 17% of normal activity whereas the mitochondrial activity of this enzyme was 6% of normal. Pyruvate dehydrogenase deficiency was present in six cases showing 8 to 39% of normal activity, five of them being due to deficient pyruvate decarboxylase activity and one of them being due to deficient dihydrolipoyl dehydrogenase activity. Two cases were found with normal enzymes of pyruvate metabolism in which the production of 14CO2 from [3-14C]pyruvate was deficient at 13 and 28% of normal activity, respectively, which we consider to be indicative of reduced activity of the Krebs' cycle. The grounds for the diagnosis of these 14 affected cases are documented, and the clinical presentation of these enzyme deficiencies is assessed in the light of present knowledge about lactic acidosis.