Patients entered in the study are given TMTX for 21 days and LCV 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. It is essential to ensure that patients receive each and every dose of LCV and that LCV therapy is continued for a full 3 days after TMTX therapy has been completed or discontinued. Doses are adjusted if side effects, such as low white blood cell counts, 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.