Drugs now used in treatment for patients with AIDS show some toxicity which limits their usefulness. In addition, with long-term treatment with AZT, there is evidence of virus resistance to the drug. Compounds that are more effective and less toxic than those in present use would be beneficial, especially if they are active against AZT-resistant viruses. Nevirapine has shown in vitro (test tube studies) activity in inhibiting HIV replication (reproduction). In vitro studies have shown that nevirapine and AZT work together to inhibit HIV replication.