A method is described to purify fumarylacetoacetase from crude human liver extracts using immunoaffinity chromatography. Immobilized rabbit antibodies specific for beef liver fumarylacetoacetase were used as an immunoadsorbent. With this rapid and specific procedure human liver fumarylacetoacetase could be purified to apparent homogeneity. The molecular weight of native human liver fumarylacetoacetase is approximately 83000 as estimated by gel filtration. The two subunits have a molecular weight of approximately 41000, as determined by sodium dodecyl sulphate polyacrylamide gel electrophoresis. Purified human liver fumarylacetoacetase has a broad pH optimum with a maximum at pH 7.2 and a Km = 2.1 microM towards fumarylacetoacetate.