Experimental colonization by Ureaplasma urealyticum in the vagina of Balb/c mice was greatly enhanced by administration of estrogen, whereas it was not affected by treatment with progesterone. Ureaplasma was recovered from almost half of the estrogen-treated animals 7 days after inoculation, and the organism persisted in all of these animals until 21 days after inoculation; on the other hand, the organism was isolated in less than 20% of the untreated, progesterone-treated, or danazol-treated mice 7 days after inoculation and was no longer isolated in any of these groups 21 days after inoculation. At least a hundredfold more organisms were recovered from the estrogen-treated mice as compared to the untreated mice 14 days after inoculation and thereafter. U. urealyticum was also isolated from the uterine cavity and salpinx in approximately half of the mice in which the organism was isolated from the vagina.