Mammary cancer occurred in five of 80 ICR Swiss mice and two of 52 phenotypically normal pituitary dwarf strain mice following intravaginal applications of 20-methylcholanthrene in doses totalling 2.0 to 5.5 mg. Twenty-two of these 132 methylcholanthrene-treated mice had lower genital tract cancer. One acetone-treated Swiss mouse developed mammary cancer. No genital tract or mammary cancer occurred in mice receiving less than 2.0 mg. methylcholanthrene. The carcinogen was administered when mice were young; mammary cancers appeared between 65 and 167 days after methylcholanthrene application was begun. This study provides additional examples of carcinogenesis in organs remote from sites of carcinogen treatment with carcinogen doses close to threshold levels for treated tissues.