Pyruvate dehydrogenase complex (PDC) activity in human skin fibroblasts appears to be regulated by a phosphorylation-dephosphorylation mechanism, as is the case with other animal cells. The enzyme can be activated by pretreating the cells with dichloroacetate (DCA), an inhibitor of pyruvate dehydrogenase kinase, before they are disrupted for measurement of PDC activity. With such treatment, the activity reaches 5-6 nmol/min per mg of protein at 37 degrees C with fibroblasts from infants. Such values represent an activation of about 5-20-fold over those observed with untreated cells. That this assay, based on [1-(14)C]pyruvate decarboxylation, represents a valid measurement of the overall PDC reaction is shown by the dependence of (14)CO(2) production on the presence of thiamin-PP, coenzyme A (CoA), Mg(++), and NAD(+). Also, it has been shown that acetyl-CoA and (14)CO(2) are formed in a 1:1 ratio. A similar degree of activation of PDC can also be achieved by adding purified pyruvate dehydrogenase phosphatase and high concentrations of Mg(++) and Ca(++), or in some cases by adding the metal ions alone to the cell homogenate after disruption. These results strongly suggest that activation is due to dephosphorylation. Addition of NaF, which inhibits dephosphorylation, leads to almost complete loss of PDC activity. Assays of completely activated PDC were performed on two cell lines originating from patients reported to be deficient in this enzyme (Blass, J. P., J. Avigan, and B. W. Ublendorf. 1970. J. Clin. Invest. 49: 423-432; Blass, J. P., J. D. Schuman, D. S. Young, and E. Ham. 1972. J. Clin. Invest. 51: 1545-1551). Even after activation with DCA, fibroblasts from the patients showed values of only 0.1 and 0.3 nmol/min per mg of protein. A familial study of one of these patients showed that both parents exhibited activity in fully activated cells about half that of normal values, whereas cells from a sibling appeared normal. These results demonstrate the inheritance nature of PDC deficiency, and that the present assay is sufficient to detect the heterozygous carriers of the deficiency. Application of the same procedures to fibroblasts obtained from 16 individuals who were believed to have normal PDC activities showed a range from about 2-2.5 nmol/min per mg protein for adults to 5-6 nmol/min per mg protein for cells from infants.