Administration of oestradiol to three strains of female mice induced the oestrous phase of the reproductive cycle in which there are few or no polymorphonuclear leucocytes (PMNL) in vaginal smears. This treatment rendered the mice susceptible to genital tract colonisation by serotype 8 of Ureaplasma urealyticum, inoculated intravaginally. BALB/c mice were the most susceptible, all of 10 becoming colonised; two other strains were less susceptible and untreated mice were resistant. The numbers of ureaplasmas recovered from the vagina ranged from 10(2) to 10(7) colour-changing units (ccu)/ml, irrespective of the strain of mice, and in some there was spread to the uterine horns and ovaries, and to the spleen in one mouse. Vaginal colonisation persisted for 21-163 days and subsequent failure to recover the organisms seemed to be associated with re-establishment of the oestrous cycle. There was no evidence of a genital tract PMNL response but some of the mice developed a four-fold or greater antibody response, measured by the metabolism-inhibition technique. This, however, was insufficient to protect mice against recolonization by the same serotype of U. urealyticum.