Monoclonal antibodies against the prototype 8FIIa strain of Norwalk virus were prepared and applied to an enzyme immunoassay (EIA) for detecting Norwalk virus in stool specimens. The monoclonal antibodies immunoprecipitated a 58-kDa protein which had been produced by in vitro transcription-translation of Norwalk virus cloned cDNA, and they reacted by EIA with recombinant Norwalk virus capsid protein at a sensitivity level of 1 ng/ml. The EIA detected virus in all tested samples from 15 different Norwalk virus-infected volunteers. No cross-reactions were seen in stools containing other caliciviruses or in stools containing rotaviruses, astroviruses, or enteric adenoviruses.