THE polyoma virus, originally isolated from mice, has been shown to produce multiple tumours when inoculated into new-born mice and hamsters (Stewart, Eddy and Borgese, 1958). Growth of the virus in mouse embryo tissue cultures is accompanied by a regular cytopathic effect, notably clumpino, of cells and detachment from glass surfaces (Eddy, Stewart and Berkeley, 1958). The fluorescent antibody technique has shown that in polyoma-infected tissue culturcs of mouse embryo cells viral antigenic material develops first in the host cell nuclei and appears later in the cytoplasm (Henle, Deinhart and Rodriguez, 1959). Electron micrographs of ultrathin sections of tissue culture cells infected with the prototype SE and Mill Hill strains of polyoma virus have shown large niimbers of virus-like particles, mostly in the nuclei (Bernhard, Febvre and Cramer
