Prenatal diagnosis of pyruvate carboxylase (PC) deficiency was performed in a family at risk for the acute neonatal form of this disease which manifests secondary citrullinemia. The diagnosis of an affected child was confirmed by enzyme assay and 3H-biotin labelling of proteins in cultured fetal skin fibroblasts. Sufficient amniocytes were cultured in 3-4 weeks for enzyme analysis in two centres. Citrulline concentration in amniotic fluid (AF) was normal in the affected fetus.