The mammary tumor virus (MuMTV) in the milks of 7 mouse strains and substrains was titrated for infectivity in 4 strains. The data indicated that: 1) Each strain shed a different MuMTV and some genetic strains carried two MuMTV's, each discernible by its mouse strain preference in infectivity tests. 2) Less than 5% of RIIIf and about 10% of Af mice shed detectable MuMTV antigen in their milks after the third parturition. After the sixth parturition, 33% of RIIIf and 50% of Af, and after the ninth parturition, 60% of RIIIf and 90% of Af mice shed viral antigen in their milks. The MuMTV's in milks of high-parity mothers were most infectious in mouse strains different from those most susceptible to MuMTV in RIII and A milks of low-parity mothers. Therefore, RIII and A mice each harbored two viruses, one that was removed by foster-nursing and the other that was not. 3) The susceptibility incidence of RIIIfC57BL mice to RIII virus changed gradually from about 10% in 1970 to about 70% in 1975. Susceptibility of C57BL mice to RIII virus did not change appreciably over this period, and the natural tumor incidence in RIIIfC57BL remained unchanged (about 10%). In addition to their susceptibility to RIII virus, C57BL mice were also susceptible to GR virus; they were relatively resistant to other strains tested. They were especially resistant to RIIIf virus, to which Af and BALB/c mice were very susceptible. 4) Approximately 90% of C3HfC57BL and C3HfBALB/c mice shed antigen in their milks after the third parturition, although the tumor incidence was less and occurred later than in C3H mice. No clear-cut differences could be detected in infectivities between low-parity C3H milk and high-parity C3Hf milks tested in several assay strains.