A new human adenovirus has been isolated from 62 eyes with (kerato)conjunctivitis and from nine genitourinary sites. The virus is closely related in haemagglutination inhibition tests to adenovirus type 19 (Ad 19) and Ad 10. Antiserum adsorption experiments demonstrated the presence of three haemagglutinin antigens in the virus: One unique, another common to Ad 19, and a third common to Ad 10 and 19. In neutralization tests, the virus is distantly related to Ad 13, 30, 19, and 10. Despite this relationship, it is proposed to call the virus adenovirus 37, in agreement with current species definitions. It belongs to subgroup D of human adenoviruses. Antisera to the new virus show virtually no neutralization of other human adenovirus types. Only bay use of this antiserum it is in practice possible to avoid wrong or indefinite typing, which has often occurred in the past.