1. The inverse relationship between vaginal sensitivity to estrogen and mammary tumor incidence that has been reported for limited numbers of strains of mice was found not to apply when larger numbers of strains were studied. This was the case when either all strains or only those known to possess milk factor were considered.

  2. The presence or absence of milk factor, obtained by foster-nursing or by reciprocal crossing of a high and a low tumor strain, did not alter vaginal sensitivity to estrogen.

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