Deficiency of holocarboxylase synthetase leads to multiple carboxylase deficiency, which is fatal in the absence of prompt diagnosis and treatment with biotin. In a pregnancy at risk for deficiency of holocarboxylase synthetase, prenatal diagnosis was performed by assay of the enzyme in chorionic villus material. The Km for biotin was 220.8 nmol/l, which was 33 times the control value of 6.6 nmol/l. Biotinyl AMP synthesis was undetectable in cultured chorionic villus material. Prenatal treatment of the mother was begun with 10 mg a day of biotin and continued through pregnancy. There was no accumulation of the characteristic metabolites in the urine at birth and prior to oral treatment of the newborn. Holocarboxylase synthetase activity was undetectable in lymphocytes and in fibroblasts of the newborn. Furthermore, the activities of all three carboxylases in fibroblasts of the infant were deficient. The newborn was clinically well and maintained on biotin treatment after birth at 20 mg per day. Carboxylase activities in lymphocytes were normal or slightly lower than the normal range.