The incidence of mammary tumors in germ-free C3H mice.

The removal of the Bittner virus by foster nursing C3H mice on mice that do not have the agent (e.g., C57BL) postpones the development of mammary tumors, but does not prevent it (1, 3-6, 8). All inbred strains of germ-free mice presently in existence have been derived by foster nursing their young on descendents of Swiss mice which were derived by hand feeding by the Notre Dame group (13). All germ-free mice, therefore, are presumed to be free of the milk-transmitted Bittner mammary tumor virus. Workers in the Notre Dame group (14, 15) have reported a failure to observe mammary tumors in germ-free C3H mice. Recently, however, Kajima and Pollard (7) have reported "B particles" in two germ-free C3H mouse mammary tumors. They state that, "A large colony involving generations of germ-free