The combination of clindamycin and primaquine was investigated as primary treatment for mild or moderately severe Pneumocystis carinii pneumonia (alveolar-arterial oxygen gradient less than 40 torr) in 36 AIDS patients. All patients were treated with primaquine 30 mg (base) orally for 21 days. The first 22 patients were treated with clindamycin 900 mg i.v. q8h for the first 10 days, followed by clindamycin 450 mg q6h orally for 11 days. Twenty of these 22 patients (91%) showed a marked clinical response by day 7. Four patients were later withdrawn from the study due to toxicity. An additional 14 patients were treated with an entirely oral regimen of clindamycin and primaquine. Thirteen of the 14 patients (93%) showed a marked therapeutic response and 12 (86%) successfully completed treatment. The combination of clindamycin and primaquine appears to be an effective and well-tolerated treatment for mild to moderately severe Pneumocystis carinii pneumonia in AIDS patients.