Fumarylacetoacetase was assayed in cultured amniotic fluid cells from four pregnancies at risk for hereditary tyrosinemia and in 11 controls. The enzyme activity was normal in three of the pregnancies at risk for tyrosinemia and healthy children were born. In the fourth case the enzyme activity was deficient, indicating an affected fetus. As the pregnancy was very advanced it was continued, and the child has tyrosinemia. One parent in one of the four families is a compound heterozygote for the tyrosinemia gene and the recently reported "pseudodeficiency" gene for fumarylacetoacetase. This has important consequences for prenatal diagnosis in this family.