A Study to Compare the Safety and Effectiveness of Two Dosing Schedules of Lamivudine in Combination With Two Other Anti-HIV Drugs

Inclusion Criteria Patients may be eligible for this study if they: * Are HIV-positive. * Are at least 18 years old. * Have had an HIV level below 400 copies/ml for at least 3 months prior to study entry. * Have a CD4 cell count of at least 50 cells/mm3. * Are currently taking an anti-HIV drug regimen that includes 3TC plus d4T plus either IDV or NFV for at least 6 months prior to study entry. (Note: This must be their first anti-HIV drug regimen.) * Agree to abstain from sex or use effective methods of birth control during the study. Exclusion Criteria Patients will not be eligible for this study if they: * Have a history of an AIDS-defining illness or certain other medical conditions. * Are allergic to any of the study drugs. * Are unable to take medication by mouth for any reason. * Have received certain medications. * Will need to receive radiation therapy or chemotherapy (for any cancer other than Kaposi's sarcoma) during the study. * Are pregnant or breast-feeding.