To determine if the pharmacokinetics of low doses of zidovudine (AZT) (that is, how fast AZT reaches the blood, what concentration of AZT is attained in the blood, and how long AZT remains in the blood) changes from day-to-day in the same patient. Also to determine whether the pharmacokinetics of AZT is changed by sulfamethoxazole/trimethoprim (SMX/TMP) given at the same time or whether the pharmacokinetics of SMX/TMP is altered by AZT therapy. AZT has been effective in treating some patients with AIDS, and SMX/TMP is an antibiotic combination which is useful in preventing or treating Pneumocystis carinii pneumonia (PCP), which is an important cause of disease and death in patients with AIDS. It is important to know how drugs interact in patients because addition of a second drug may change the speed at which a drug is eliminated from the body, and cause increased toxic effects or decreased therapeutic effects.