This is an open-label, phase I study examining the safety and anti-HIV activity of an investigational non-nucleoside reverse transcriptase inhibitor, capravirine. The primary objectives of the study are 1) To evaluate the safety and pharmacokinetics of capravirine when combined with efavirenz and (low-dose) ritonavir in non-HIV-infected volunteers; 2) To evaluate the safety, virologic activity, and pharmacokinetics of capravirine when combined with efavirenz, (low-dose) ritonavir, and abacavir in HIV-infected patients; and 3) To evaluate the safety, virologic activity, and pharmacokinetics of a once-daily regimen with capravirine, efavirenz, (low-dose) ritonavir, and abacavir in HIV-infected patients. The study will be conducted in 2 parts. Part I will examine the safety, pharmacokinetics, and potential drug interactions of capravirine with other antiretroviral agents in non-HIV-infected volunteers. Sixteen volunteers (8 per group) will receive capravirine monotherapy (7 days), followed by the addition of either: 1) efavirenz (10 days); or 2) ritonavir (8 days). Part II will examine the safety, virologic activity, pharmacokinetics, and potential drug interactions of capravirine in HIV-infected patients. Thirty patients (10 per group) will receive of capravirine monotherapy (7 days), followed by the randomized addition of: 1) efavirenz/abacavir (7 days); 2) ritonavir/efavirenz/abacavir (7 days) twice-daily regimen; or 3) ritonavir/efavirenz/abacavir (7 days) once-daily regimen. [Doses of capravirine may be changed depending upon pharmacokinetic data obtained from Part I of the study.] The primary efficacy endpoint is the time-to-virologic-failure.