To determine the contribution to the efficacy of clindamycin in acute murine toxoplasmosis of treatment protocol variables, groups of Swiss Webster mice inoculated intraperitoneally with 10(2) RH strain Toxoplasma gondii tachyzoites were treated with peroral clindamycin at 25, 50 and 400 mg/kgBM per day for 1, 2 and 3 weeks. While the lowest drug dose applied for a single week prolonged survival time as compared to untreated animals, not even the highest dose applied for 1 or 2 weeks completely prevented mortality. Conversely, 100% protection was achieved with 3-week treatment courses at both 50 and 400 mg/kg per day. While both survival rates and survival times increased in parallel with the drug dose and treatment duration, the latter was shown to be critical to the outcome, suggesting the use of clindamycin as an antitoxoplasmic agent should be as a prolonged course.