The expression of mammary tumor virus (MTV) antigen in the milk and various organs of three non-inbred dd mouse stocks (ddO, ddN and ddY) was examined by the immunodiffusion (ID) and micro-immunodiffusion (micro-ID) tests. The rate of MTV antigen expression in the milk was 100% at the first lactation in dd0 (6/6) and ddN mice (10/10), and 23% in ddY mice (3/13) . Mammary tumor incidence was 13% (mean tumor age : 12.0 months), 32% (9.6 months) and 10% (11.5 months) in ddO, ddN and ddY mice, respectively. In F₁hybrids between MTV-free BALB/c females and dd males, a high level of MTV antigen was detected by the ID test in the milk of (BALB/c×ddO) F₁, however, the levels in (BALB/c×ddN) F₁and (BALB/c×ddY) F₁mice were low at the first lactation and elevated with the advance of lactation number. Mammary tumor incidence had a trend to be higher and earlier in these F₁hybrids than in non-inbred dd stocks. The development of mammary tumors and detection of MTV antigen in F₁hybrids indicate the extrachromosomal transmission of MTV by male dd mice. The micro-ID test has shown that the mammaty tumors, mammary glands, male genital organs except for the testis and the salivary gland expressed MTV antigen, with a high frequency of suggesting that secondary male genital organs may play an important role in MTV infection in mice.