Toxicity required discontinuation of therapy in approximately 5 percent of all patients. Original design: Patients entered in the study are given TMTX once a day for 21 days and LCV 4 times a day (every 6 hours) for 24 days. Doses are determined by body size. Both drugs are given by intravenous infusion, but LCV may be given orally after the first 10 days. Doses are adjusted if side effects, such as low white blood cell count, are too severe. During the 21-day trial, zidovudine (AZT) may not be used, because of possible increased bone marrow toxicity. AZT may be resumed as soon as the administration of TMTX and LCV has been completed. After treatment with TMTX, the patient may be treated with other drugs to prevent the recurrence of PCP at the discretion of his/her physician.