Nine viruses antigenically related to Hantaan virus that were isolated from various rodents in several countries and from a patient were examined by immunoprecipitation and by hemagglutination inhibition (HI). The immunoprecipitation tests were done with a monoclonal antibody that reacted with glycoprotein G2 and an antibody against a nucleocapsid protein. The viruses were classified into four subtypes on the basis of the molecular weights of their precipitated polypeptides and the pattern of results of the HI tests.