Treatment of female BALB/c mice with oestradiol rendered them susceptible to vaginal colonization by three of four different strains of Mycoplasma hominis. Overall, the organisms were recovered persistently from the vagina of 68 (87%) of 78 of these mice. Strain TO mice given one of the strains were at least susceptible, all of ten becoming colonized and larger numbers of organisms being recovered. The hormone arrested the reproductive cycle in the oestrous phase, characterized by non-nucleated, cornified vaginal epithelial cells. In contrast, M. hominis organisms were isolated transiently from only seven (10.5%) of 66 BALB/c mice not treated with oestradiol, after intravaginal inoculation; treatment with progesterone, which induced the dioestrous phase of the cycle, did not render any of 10 BALB/c mice susceptible to vaginal colonization. The minimum number of organisms (2.5 x 10(5)) of one strain of M. hominis and the minimum dose of oestradiol (0.05 mg) required to induce persistent colonization were established. Vaginal colonization persisted for more than 200 d in some mice, the numbers of organisms recovered ranging between 10(1) and 10(8). At autopsy there was evidence of spread to the uterine horns and ovaries, and also to the oropharynx, of some animals but not to other organs. Infection was not associated with a polymorphonuclear leucocyte response in the vagina or elsewhere, but a fourfold serum antibody response to M. hominis, measured by the metabolism-inhibition technique, was detected in almost half of the mice tested.