Abstract A radiochemical assay for adenylosuccinase, an enzyme which intervenes twice in the biosynthesis of adenine nucleotides, has been developed. The two substrates of the enzyme, succinylaminoimidazole car☐amide ribotide (SAICAR) and adenylosuccinate (S-AMP), were synthesized in radioactive form by incubating [2,3- 14C]fumarate and, respectively, AICAR and AMP with partially purified adenylosuccinase from yeast. Enzyme activities were determined by measuring the release of labeled fumarate after its separation from the substrate by chromatography on polyethyleneimine thin-layer plates. The ratio of the activity of adenylosuccinase measured with SAICAR compared to that with S-AMP was about 1 in crude extracts of rat liver and muscle and around 0.5 in human liver. In rat and human liver, but not in rat muscle, 20 to 40% of both activities of adenylosuccinase were lost after freezing at −80°C followed by thawing. In the liver of patients with adenylosuccinase deficiency, in whom the deficiency had hitherto been measured only with S-AMP, the activity of the enzyme toward S-AMP and SAICAR was found to be lost in parallel. This is in accordance with the finding that both SAICA-riboside and succinyladenosine accumulate in adenylosuccinase-deficient patients.