Cancer has been caused by several factors, such as : Virus : biological factorRadiation : physical factorCarcinogenic chemical compound : chemical factor. The study of the transmission of MTV on experimental animals and its descendants has been tried since 1963. Several strains of mice have been bred successfully, one of which is the C3H strain. This strain is known to'have a high mammary tumor incidence due to the high content of the MTV (Bittner virus) in mother's milk. The transmission of MTV can be observed by foster nursing or reciprocal crossing between strains with high and low mammary tumor incidence. All females used in this experiment were force bred and observed for the appearance of a tumor until the age of one year. Some conclusions from this experiment are that: MTV was transmitted through mother's milk.Other strains (Balb/c and AJ) could develop mammary tumor when infected by MTV.There is a different susceptibility between Balb/c and AJ to MTV. The transmission of MTV from parent to off spring is -called the vertical transmission. This virus has been known as Bittner virus and it can be identifed by the electrone microscope, immunodiffusion test (Ouchterlony) or immunofluorescence test. The result of this experiment may be useful for studying the problem of Cancer tumor in human beings.