Three serovars (5, 8 and 10) of Ureaplasma urealyticum were inoculated intravaginally into groups of oestradiol-treated young adult BALB/c strain mice. Hormone treatment was essential for vaginal colonisation. The proportion of mice colonised initially and the persistence of colonisation were different with the three serovars; half of those given serovar 8 were still colonised after 84 days. A strain of serovar 5 after a further 50 subcultures in vitro was a little less persistent than it was before such subculture, but not in a way to suggest that subculturing was the main reason for differences in the behaviour of the serovars. At autopsy of six mice that were still colonised vaginally 158 days after inoculation of serovar 8, spread to the upper genital tract was shown to have occurred in three of them and dissemination to the liver and kidney in one. Compared with immunocompetent mice of strain CB20, such dissemination was not a feature in genetically related mice with severe combined immunodeficiency. This is not in keeping with the situation in hypogammaglobulinaemic patients in whom ureaplasmas and other mycoplasmas are known to disseminate. However, differences in the proportion of immunocompetent mice colonised or in ureaplasmal persistence with different serovars may act as a marker for differences in human pathogenicity and is worthy of further study.