Mandelonitrile beta-glucuronide, the compound patented as Laetrile, has been synthesized from rabbit liver uridine diphosphate-glucuronosyl transferase immobilized on beaded sepharose, has been analyzed by thin-layer chromatography, nuclear magnetic resonance, and gas chromatography-mass spectrometry, and has been tested for cytotoxicity and mutagenic activity with Salmonella typhimurium strains TA 98 and TA 100. Several commercial laetrile preparations contained no glucuronide; they contained amygdalin and neoamygdalin instead. Mandelonitrile, mandelonitrile glucuronide, and a mixture of amygdalin and neoamygdalin were each found to be mutagenic.