Patients with HIV infection take AZT every 4 hours and/or SMX/TMP every 8 hours by mouth for 4 days as outpatients and then come into the clinical research center for 2 days of studies. On day 5 the final dose of medicine is given orally SMX/TMP or by intravenous infusion (AZT). Blood samples are drawn 10-20 times over a period of 12 hours and urine is collected for 36 hours. Concentrations of the drugs in the blood and urine samples are determined. This sequence is repeated twice, so that each patient takes AZT alone, SMX/TMP alone, and the combination of AZT and SMX/TMP over a period of about 3 weeks. Patients may be included in the study if they are asymptomatic, or have been diagnosed with ARC or AIDS, but not if they have PCP or any other severe opportunistic infection.