Although many serologic tests are available for determination of antibody to Toxoplasma gondii, circulating antigen has not been studied in infections with this organism. Presence of circulating antigen was sought by immunologic methods in experimental infections (Rh strain) of mice and rabbits. In mice, which succumbed to infection within four days; circulating antigen was detectable in serum by counter-current immunoelectrophoresis and agar gel diffusion on days 2-4 of infection. In rabbits; which succumbed to infection within eight days; serum antigen could not be detected by countercurrent immunoelectrophoresis or agar gel diffusion; Affinity chromatography, with use of cyanogen bromide-activated Sepharose and binding of the antigen to hyperimmune rabbit antiserum to Toxoplasma, permitted isolation of serum antigen on days 3, 5, 7, and 8 of infectionmalthough infected micce and rabbits may have parasitemia of 10-2--10-4 organisms/ml of blood, this concentration did not produce precipitin reactions with antiserum that detected antigenemia. Preliminary characterizations of the column-extracted antigen revealed heat inactivation by 56 C for 30 min, complete inactivation by trypsin, and a molecular weight of greater than 100,000 daltons, as determined by chromatography on a Sephadex column.