The detailed studies of the Abney type 3 strain of reovirus that warrant its inclusion in this group are described. The cytopathic effect is typical although it appears more slowly than with types 1 or 2. Titres as high as 10 8 are obtained but titration tubes have to be held for 3 weeks and then subcultured to determine if virus has grown. The virus is ether resistant and the same size as other reoviruses. It is pathogenic for suckling mice by the intracerebral, intraperitoneal or subcutaneous routes, the chief lesions being in the liver. The viruses were isolated from children in an institution (Junior Village) in Washington. The first 3 were isolated during October 1955 to February 1956 when 26 out of 34 children in the nursery had serological evidence of reovirus type 3 infection. Later, in July to October 1957, there was another episode of reovirus type 3 infection in the nursery, 19 out of 68 children giving serological evidence of infection. In this outbreak 32% of the children had evidence of prior infection with the virus. During this second outbreak 8 isolations of virus were made from anal swabs and also from throat swabs of 2 of the children. 1 of the children was excreting virus on the day of admission and was therefore not considered to have been infected in the institution. It did not prove possible to identify a clinical syndrome associated with reovirus type 3 infection but for 6 of the children sufficiently well studied, from whom a virus was isolated, 5 suffered from a febrile illness within a 9-day period bracketing the virus isolation. It is at least possible that reovirus infection is manifested by fever, coryza and diarrhoea, but these symptoms are so common in this study population that this is no more than a possibility. AJ Beale.