Two healthy adults with low fumarylacetoacetase activity in fibroblasts and lymphocytes, one a compound heterozygote for the tyrosinaemia and the pseudodeficiency genes and the other a homozygote for the pseudodeficiency gene, produced substantial amounts of succinylacetone when given an intravenous homogentisate load. The level of metabolites correlated with the residual enzyme activity and the genotype, being higher in the compound heterozygote. This subject also showed a small increase of metabolites in urine after an oral tyrosine load. In the pseudodeficiency homozygote a depression of erythrocyte delta-aminolevulinate dehydratase activity was observed after the tyrosine load. In fasting state both individuals have erythrocyte delta-aminolevulinate dehydratase activity below the reference range, indicating a persistently raised concentration of metabolites. Thus, the pseudodeficiency state is not just an in vitro phenomenon, but results in a definite reduction of enzyme activity in vivo. We speculate that the variant gene may predispose to the development of liver disease, possibly not recognized as tyrosinaemia.