The chemotherapeutic efficacy of trimethoprim-sulfamethoxazole ($Bactrim^{circledR}$) in mice experimentally infected with Toxoplasma gcndii was evaluated. The average survival days and survival rate of mice infected intraperitoneally with $1{ imes}10^5$ trophozoites and treated with $Bactrim^{circledR}$ were compared with those of untreated group. The hematologic findings of blood samples of experimental mice were observed for comparison of side elects between $Bactrim^{circledR}$ and pyrimethamine ($Daraprim^{circledR}$), the latter of which has been one of the favorable drugs for the treatment of toxoplasmosis. The results are summarized as follows: 1. $Bactrim^{circledR}$ showed a strong evidence of potent anti-Toxoplasma activity. The survival rate of mice administered with 24 mg of $Bactrim^{circledR}$ per mouse per day for 7 days, was 83.3%, and the rate was increased to 100% in mice administered with two-fold concentrated dose of the drug. 2. The average numbers of white blood cells (W.B.C.) in the mouse groups treated with $Bactrim^{circledR}$ or $Daraprim^{circledR}$ were more increased than those only infected with T. gondii. The mice treated with $Daraprim^{circledR}$ however, showed remarkably decreased numbers of W.B.C. as compared with those treated with $Bactrim^{circledR}$ . 3. The average numbers of red blood cells (R.B.C.) and platelets both in the drug-treated and untreated T. gondii-infected mice were decreased as compared with normal mice. The numbers of R. B. C. in $Daraprim^{circledR}-treated$ mice, however, were more decreased than in $Bactrim^{circledR}-treated$ mice. 4. The average levels of hemoglobin both in the drug.treated and untreated T. gondii-infected mice were decreased, compared with normal mice. But there was no difference in the levels of hemoglobin between $Bactrim^{circledR}$ and $Daraprim^{circledR}-treated$ groups. In conclusion, trimethoprim.sulfamethoxasole ($Bactrim^{circledR}$) was proven to be effective and safe for the treatment of murine toxoplasmosis. The efficacy was comparable with pyrimethamine ($Daraprim^{circledR}$), but bone marrow depression was less severe with $Bactrim^{circledR}$ treatment.