Human sera obtained from two sources in Baltimore were tested for antibodies against a Hantavirus by a variety of techniques. Four persons out of 1,788 had hantaviral antibody as demonstrated by immunofluorescent assay, enzyme-linked immunosorbent assay, and neutralizing assay. Neutralizing antibody titrations against three test viruses indicated that infections were caused by a rat-associated virus. Medical histories of two subjects indicated life-long residence in Baltimore and an absence of foreign travel, implicating a local source of infection.