An unusual form of ornithine transcarbamylase deficiency was found in a male child who became unconscious at 8 months. Two maternal uncles had died during similar illnesses at 6 years and 11 years, respectively. Detailed studies of the enzyme showed 10% residual activity, a very low Km for carbamyl phosphate (0.015 mmol/l) and near normal amounts of immunoreactive protein with a smaller than normal subunit (molecular weight 37 800 instead of 39 700). This information was obtained from a 10 mg liver biopsy core using protein separation on SDS-polyacrylamide gel, electrophoretic transfer to nitrocellulose filters and probing with antibody to the enzyme. Resolution of the exact mutation causing this change will be of interest to those who are studying the processing of mitochondrial enzymes during transport from the cytoplasm.