Fasciola hepatica adult worm cysteine proteinases were active-site, affinity radio-labeled with benzyloxicarbonyl-L-tyrosine-L-alanine diazomethylketone (Z-Tyr125I-Ala-CHN2). Sera from patients with fascioliasis and from rabbits experimentally infected with F. hepatica immunoprecipitated the radiolabeled parasite cysteine proteinases in immunoelectrophoresis assays. Two purified antigens were identified as part of the complex mosaic of antigens immunoprecipitated by the sera of infected patients. These antigens (Fas1 and Fas2) have been shown to be an important part of the Fharc2 precipitin band used for serologic diagnosis in humans and cattle. They showed cysteine proteinase activity with different proteolytic specificities and partial identity in double immunodiffusion assays. The results obtained in this work show that the Fas1 and Fas2 antigens are sensitive and specific antigens for diagnosis of this serious helminthic disease in humans and other susceptible hosts.