A Randomized, Unblinded Trial of Zidovudine Versus ddC in the Treatment of Patients Status Post PCP Who Received Long-Term Zidovudine Therapy in Protocol ACTG 002

Inclusion Criteria Required: * Prior zidovudine (AZT) therapy for 9 months. Concurrent Medication: Allowed: * Chemoprophylaxis for Pneumocystis carinii pneumonia (PCP) with aerosolized pentamidine of 300 mg every 4 weeks through the Respirgard II nebulizer. * Maintenance treatment with pyrimethamine, sulfadiazine, amphotericin, fluconazole, ketoconazole, acyclovir, or inhaled pentamidine for subjects who have recovered from toxoplasmosis, cryptococcosis, candidiasis, herpes infection, or PCP. * Dapsone for PCP. * Pyrimethamine-sulfadoxine for toxoplasmosis. * Ganciclovir (DHPG) for maintenance only for cytomegalovirus (CMV) retinitis. * Note: Any approved medications can be used to treat an opportunistic infection. All concurrent medications should be kept to a minimum and recorded. Patients must be positive for HIV by ELISA test and must have been receiving zidovudine (AZT) therapy for at least 9 months and have received AZT within 90 days prior to entry into the study. Patients may be transfusion dependent as long as no more than 3 units of blood are needed in a 21-day period and the hemoglobin does not fall below 6.4 g/dl on two consecutive occasions despite the transfusions. Exclusion Criteria Concurrent Medication: Excluded: * Antiretroviral study medications other than zidovudine (AZT) and biologic response modifiers. * Corticosteroids and chronic aspirin. * Cimetidine. * Flurazepam. * Indomethacin. * Ranitidine. * Probenecid. * Other experimental medications. Patients will be excluded from the study for the following reasons: * Removal from zidovudine (AZT) during treatment on ACTG protocol 002 for recurrent grade 4 toxicity. * Removal from prior dideoxycytidine (ddC) therapy for peripheral neuropathy = or \> grade 3. * Visceral or extensive Kaposi's sarcoma requiring therapy or another malignancy requiring therapy. * Toxicity grades according to NIAID Recommendations for Grading Acute and Subacute Toxic Effects (Adults). Prior Medication: Excluded: * Antiretroviral study medications other than zidovudine (AZT) and biologic response modifiers. * Patients may not have visceral or extensive Kaposi's sarcoma requiring therapy or another malignancy requiring therapy.