Abstract: This paper reports on the regular pattern of growth and decline of the circulating antigen (CA) determined by I-ELISA in experimentally infected rabbits. Toxoplasma antigen was demonstratable in the circulation in 64.7% rabbits 1 day after tachyzoite inoculation and the level of the CA in sera from infected rabbits showed an increase on d4 and reached its peak level on d10-13 after inoculation but declined soon thereafter. CA was not detectable on days 60 to 90 after inoculation, The results suggest that determining CA is a useful means for diagnosing active toxoplasmosis and that I-ELISA might be a promising approach to detect CA in toxoplasmosis.