Colonization of the genital tract of female mice, mainly BALB/c, by Mycoplasma genitalium, M. pneumoniae and M. pulmonis was enhanced by pretreatment of the mice with progesterone. M. fermentans, M. hominis and M. salivarium, and three serotypes and two untyped strains of Ureaplasma urealyticum colonized under the influence of oestradiol but not progesterone. Mycoplasmas dependent on progesterone were glucose-metabolizing, with strong haemadsorptive and other attachment properties, and possessed a terminal structure. Mycoplasmas dependent on oestradiol were arginine or arginine/glucose metabolizing. Ureaplasmas also required oestradiol. The oestradiol-requiring group appeared to be less cytadsorptive and devoid of a morphological terminal structure. Mycoplasmas that had had multiple passes in media were less able to colonize. This may be one, but not the only, reason for the failure of seven mycoplasmas to colonize under the influence of either hormone. The observations suggest the existence of a receptor mechanism for colonization, progesterone-requiring mycoplasmas being exposed to, and needing, genital tract cells different from those exposed to oestradiol-requiring mycoplasmas.