Urine samples collected from three groups of mice infected experimentally with different numbers doses of Toxoplasma trophozoites were detected for the presence of Toxoplasma circulating antigen by using fast ELISA. The results showed that presence of circulating antigen in all three infected groups of mice in comparison with the normal control group. Toxoplasma circulating antigen was detected on days 5, 4 and 3 after infection in light-, moderate- and heavy-infection groups, respectively. The concentration of circulating antigen was on a parallel with the duration of infection. Western blot analysis of the Toxoplasma circulating antigen in urine revealed the existence of seven specific bands with molecular weights of 75, 67, 55, 43, 30, 28 and 22 kDa.