Rattus (Mastomys) natalensis, the multi-mammate mouse, is an African rodent intermediate in size between the rat and the mouse. These animals have been used in microbiological studies because they are highly susceptible to infection by plague bacilli1, Schistosoma mansoni 2, and Borrelia duttonii 3. They have recently been used in cancer research laboratories following Oettle's report of a high incidence of spontaneous adenocarcinoma of the stomach in a Mastomys colony in Johannesburg, South Africa4. Since the polyoma virus has been shown to produce tumours in mice, Syrian hamsters, rats and rabbits5, we were interested in determining the response of Mastomys to this agent.