Objective To investigate the pathogenicity of the same concentration of Ureaplasma urealyticum serotype1 (Uu1) and 3 (Uu3),alone or in combination,in genital tract of female BALB/c mice.Methods A total of 144 estradiol-pretreated adult female BALB/c mice were randomly and equally divided into 3 groups to be inoculated intravaginally with the same concentration (106 copies/g) of Uu1 and Uu3 alone or in combination.Twelve estradiol-pretreated female BALB/c mice were inoculated with sterile culture solution and served as the control group.At 1,3,7,14,21,35 days after the inoculation,8 mice in each Uu-inoculated group and 2 mice in the control group were sacrificed.Tissue specimens were obtained from the cervix,endometria and fallopian tube and subjected to hematoxylin-eosin staining followed by light microscopy.Scanning electron microscopy was used to observe the tissue specimens collected at 14 days from cervical mucosa of mice infected by Uu.Chi-square test was performed for statistical evaluation.Results The total infection rate of Uu within 3-35 days after the inoculation was 35.0％ (14/40),47.5％ (19/40) and 62.5％ (25/40) in the Uu1 group,Uu3 group,and combination group respectively (x2 =6.07,P ＜ 0.05).None of these mice was infected by Uu in the control group.Conclusions At the same inoculation concentration,the combination of Uu1 and Uu3 shows the strongest pathogenicity in genital tract of BALB/c mice,followed sequentially by Uu3 and Uu1 alone.