Observations on the question of horizontal transmission of mouse mammary tumor virus

Antigen and tumor incidences in BALB/c and C57BL mice after living as weanlings for 5 weeks in cages with mouse mammary tumor virus-infected females were compared with control BALB/c and C57BL mice living in the same laboratory. All mice were bred continuously, and third-lactation milks were tested for mouse mammary tumor virus antigen by Ouchterlony microimmunodiffusion test. Mammary tumor incidences in the cagemates were not significantly different from those in the controls, although the antigen incidences were significantly greater. However, phosphate-buffered salt solution (0.02 M phosphate, pH 7.4; 0.15 M NaCl; and 0.1% bovine serum albumin) and sham-inoculated mice also had elevated antigen incidences. Repeat tests of milks at the fourth or fifth lactations indicated that more than 50% of those positive at the third became negative at later lactations.