Mild to moderately severePneumocystis carinii pneumonia in patients with AIDS was treated in a clinical trial with a combination regimen of primaquine and clindamycin, and the efficacy of this regimen was compared with that of the conventional treatment regimen of trimethoprim/sulfamethoxazole. The results revealed that primaquine/clindamycin appears to be an equally effective alternative to trimethoprim/sulfamethoxazole. The spectrum of side-effects was similar for the two regimens; side-effects occurred with equal frequency but appeared to be less severe in patients given primaquine/clindamycin. Because therapy with primaquine and clindamycin was limited to patients with mild to moderatePneumocystis carinii pneumonia, studies with this regimen in more severe cases are warranted.