A Six-Month Safety and Antiviral Study in HIV-1 Seropositive, AZT-Experienced Patients With CD4 Counts Less Than or Equal to 50 Cells/mm3 to Evaluate MK-639 Alone Versus Zidovudine (AZT) and 3TC Versus the Combination of MK-639 With AZT/3TC

Inclusion Criteria Concurrent Medication: Allowed for all patients: * Standard prophylaxis for opportunistic infections. * Continuation of treatment for opportunistic infection. Allowed for open-label study patients: * Rifampin. Patients must have: * HIV positivity. * CD4 count \<= 50 cells/mm3. * More than 6 months of prior AZT (blinded study only). NOTE: * Patients on the open-label study must have AZT intolerance or have \< 6 months of prior AZT. Prior Medication: Required for blinded study patients: * \> 6 months of prior AZT. Required for open-label study patients: * \< 6 months of prior AZT. Allowed for open-label study patients: * Prior 3TC. Exclusion Criteria Concurrent Medication: Excluded in all patients: * Immunosuppressants. Excluded in blinded study patients: * AZT, ddI, ddC, or d4T. * Rifampin. Excluded in open-label study patients: * 3TC. Prior Medication: Excluded in all patients: * Prior protease inhibitors. * Investigational agents and immunomodulators within 30 days prior to study entry. * Immunosuppressants within 2 weeks prior to study entry. Excluded in blinded study patients: * Any prior 3TC. * AZT, ddI, ddC, or d4T within 2 weeks prior to study entry. Excluded in open-label study patients: 3TC within 30 days prior to study entry.