The prenatal diagnosis of ornithine transcarbamylase deficiency (OTCD) was made by using radioactive microassays for ornithine transcarbamylase (OTC) and--as an internal control--carbamyl phosphate synthetase (CPS I) in liver biopsy material from two 19-week-old at-risk fetuses. In each case, no OTC activity could be detected, while CPS I activity was normal. Control fetuses of 17-21 weeks gestation had OTC activities in the range of 10.7 to 19.4 mumol/mg protein per hr. The prenatal results were confirmed post abortum by the radiochemical assays and by an enzyme-specific cytochemical staining method.