We have developed a method for rapid differential diagnosis of isolated or multiple deficiencies of the 3 mitochondrial biotin-dependent carboxylases: propionyl-CoA (PCC), 3-methylcrotonyl-CoA (MCC) and pyruvate carboxylase (PC), and for simultaneous evaluation of biotin-responsiveness using a single blood sample. Lymphocytes were isolated from heparinized blood and preincubated without and with 10⁻⁵ mol/1 biotin in medium before determination of PCC, MCC and PC activities. Plasma was used for estimation of biotin concentration and biotinidase activity. A definitive diagnosis could be made in 7 of 9 patients studied up to now: 4 patients suffered from biotin-nonresponsive isolated PCC-deficiency, and 3 patients from biotin-responsive multiple carboxylase deficiency caused by deficient biotinidase activity. In two patients, a carboxylase deficiency was excluded. These results were confirmed in studies using fibroblasts. In addition, a simple method for detection of deficiency in holocarboxylase synthesis is described.