Of the methods that are being evaluated to treat HIV-infected individuals, AZT has produced the best results to date. Toxic effects in approximately 50 percent of patients receiving AZT may limit its usefulness for prolonged treatment. Long-term treatment may be necessary to prevent progression of early stage HIV infection to AIDS and to prevent secondary transmission. Other drugs that may be equally or more effective than AZT and useful in the long- term treatment of HIV infection must be developed and evaluated. Test-tube and animal studies of d4T show that the drug can inhibit replication (reproduction) of HIV at concentrations similar to concentrations of AZT that have anti-HIV activity. These studies also indicate that the drug may stay in the bloodstream longer than AZT. Thus, it may be possible for the drug to be as effective as AZT when taken less frequently than AZT. It also may have a less disturbing effect on other body functions (such as thymidine metabolism).