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.

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A colony of BALB/c mice consisting of two sublines with a high incidence of mammary tumors was examined for the presence of a mammary tumor virus (MuMTV). The mammary tumor incidences in the two sublines were 18% and 35% at average tumor age 19-20 months. Over a period of 8 years, their milk at third to tenth lactations were monitored for the presence of MuMTV antigen,and the milk and tumors were examined for the presence of B particles. Neither antigen nor B particles were found. Milk and tumor extracts from the higher mammary tumor lines were also assayed for MuMTV bioactivity by intraperitoneal inoculation of weanling C57BL, BALB/c, and RIIIf females. No response was obtained, except possible in RIIIf. Both the MuMTV antigen incidence and the tumor incidence in inoculated RIIIf mice were somewhat elevated over controls. The question remains unanswered as to whether there is an active MuMTV in our colony of tumor-bearing BALB/c mice and, if there is, whether it is associated with B particles.

The occurrence rates of mammary tumors as affected by breeding regimen (early in life, late, continuous, or not at all) in A, RIII, and C3H mice were observed. The response to the breeding regimen was different in each of the three strains. The C3H stock was affected least, although the tumor occurrence rate was slower in virgins. In both A and RIII, only one litter at puberty resulted in the tumors occurring over the greatest age range; and in RIII mice, the occurrence rate and the mean tumor age were similar to those of the virgins. Normal continuous breeding caused the earliest tumors in all three strains, although the RIII mice, breeding after 18 weeks of age also caused very early tumors. The response of RIII strain to parity variations was more like that of humans than was the response of either of the other strains. Removal of the milk-transmitted virus from these strains by foster-nursing resulted in vastly different mammary tumor occurrence rates, the quantitative changes being different in each mouse strain.