As many as 80 percent of AIDS patients experience at least one episode of PCP and about one-third of these patients have a recurrence of the disease. Drugs currently used for treatment of acute PCP are toxic to the majority of AIDS patients. The combination of clindamycin and primaquine reduces the numbers of PCP organisms in laboratory tests and in animal studies. Both drugs can be given orally, concentrate in lung tissue, and have been used safely in humans for treatment of other diseases. It is possible that the combination may prove to be as good or better than standard therapy for PCP and side effects may be less.