Three cohorts of four patients each (ages 2-12 years, 3 months to less than 2 years, and 1 month to less than 3 months) receive atovaquone daily for 12 days. The oldest age group is treated first. In the absence of unacceptable toxicity, the dose of atovaquone is escalated in subsequent 4-patient cohorts representing each of the age stratifications and (per 9/30/94 amendment) in a separate 4-patient cohort aged 3 months to less than 2 years. If two of four patients in a given cohort experience unacceptable toxicity at the initial dose, two additional patients in the same age range are entered. Blood samples are drawn for pharmacokinetic evaluation. Patients are followed to day 24. Per 9/30/94 amendment, patients aged 3 months to less than 2 years of age who received one of the lower doses may re-enroll in the higher dose cohort after a 1-month washout.