Evaluation of the Interaction Between Low Dose Trimethoprim/Sulfamethoxazole and Zidovudine

Inclusion Criteria Prior Medication: Allowed: * Zidovudine (AZT) for patients with AIDS. * AIDS related complex (ARC). The presence of any one of the following findings within 12 months prior to entry and the absence of a concurrent illness or conditions other than HIV infection to explain the findings: * Fever of \> 38.5 C degrees persisting for longer than 3 weeks. * Involuntary weight loss of \> 15 lbs. or \> 10 percent of baseline noted in a 120-day period prior to evaluation. * Diarrhea (\> 2 liquid stools per day) persisting for longer than 1 month. * History of clinical diagnosis of oral candidiasis or hairy leukoplakia. * Patients who have AIDS-defining opportunistic infections or tumors. * Patients eligible for AZT under the labeling. * A positive HIV antibody test. Exceptions will be made for patients with a previously positive HIV antibody test with progressive disease and patients where virus isolation has been made. * A life expectancy of at least 3 months. * Patient with stable Kaposi's sarcoma, mild herpes infection, mild or stable depression, asymptomatic or mild cytomegalovirus or Epstein-Barr virus infection, or a hepatitis B virus carrier state will be acceptable for study. Exclusion Criteria Concurrent Medication: Excluded: * Phenytoin. Prior Medication: Excluded within 30 days of study entry: * Other antiretroviral agents. * Patient has any severe ongoing opportunistic infections including Pneumocystis carinii pneumonia (PCP), cryptococcal or toxoplasmosis meningoencephalitis, disseminated herpes simplex or herpes zoster. * Patient has significant diarrhea at entry ( \> 1 watery stool per day). * Patient has demonstrated prior sensitivity or has experienced significant adverse effects during prior therapy with the drugs to be used in the study. * Cannot abstain from alcohol or any other drugs during the study.