Abstract: Four strains of Trichomonas vaginalis were isolated from patients with different severity of symptomatology. Strains A and B produced marked symptoms in patients, strain C produced less symptoms, while strain D was isolated from carrier without symptoms. Mice- were inoculated by different routes with axenic cultures of the 4 strains of T. vaginalis, and their pathogenicity to mice were studied.All the 4 strains can induce abscess in mice. By intraperitoneal route, strains A ahd D are most pathogenic, followed by strain B and strain C, as judged from the extent of abscess formation as well as mortality rate in mice. By subcutaneous inoculation, the virulence of strains A and B is stronger than that of strains C and D, as estimated by the area of abscess produced. Subcutaneous inoculation of T. vaginalis into mice can be used as a method to estimate the virulence of various strains in human host. The size of abscess formed was found to be related to the number of protozoa inoculated.Histological sections of parasitic abscess in liver showed that it was well circumscribed, its external border was densely packed with a layer of T. vaginalis, with long axis perpendicular to the border of the abscess. Liver cells in contact with the parasitds in the border area usually showed hyaline change. Inside the abscess, necrotic liver tissues and scattered T. vaginalis could be found. Localized infiltration of mono-nuclear and polymorphic leucocytes could be seen at the border area and around the remaining portal areas in the abscess.