Patients undergo 5 weeks of antiretroviral washout before initiating therapy with ritonavir alone for 9 days, followed by combination therapy with ritonavir, zidovudine, and lamivudine from day 10 through week 48. [AS PER AMENDMENT 1/31/97: The availability of the current, open-label study treatment has been extended to allow patients who have completed 48 weeks of therapy to continue protocol therapy until the last enrolled patient completes 48 weeks of study treatment.]