Infants and children are stratified by age, representative of the developmental differences related to drug metabolism (Group I: at least 6 months - 2 years, Group II: 3-6 months, Group IIIA: 1 month - 10 weeks, IIIB: 1 month - less than 3 months). Within each age group there will be two possible dosage cohorts. All age groups will be enrolled simultaneously into dosage Cohort I, at the initial drug dosage. Progression to Cohort II (at a higher or lower drug dosage) will be decided according to safety, tolerance or viral load in Cohort I. All therapy for Group I/II, whether in Cohort I or II, will be introduced as follows: single dose of ritonavir on Day 0, ritonavir monotherapy through Day 7 AM and combination therapy from Day 7 PM through Week 104. All therapy for Group IIIA & IIIB, whether in Cohort I or II, will be introduced as follows: single dose of ritonavir on Day 0 AM and transition to combination therapy Day 0 PM through Week 104. NOTE: Progression to combination therapy for Group IIIA infants is dependent upon the results of the single-dose ritonavir pharmacokinetics (PK). If the patient is no longer at least presumed to be HIV-infected, he/she will be discontinued from the study. Replacement infants, who will not receive the single dose of ritonavir, will be acquired from Group IIIB infants; new infants that are either presumed HIV infected or have already been shown to be HIV-infected. Clinical evaluations are conducted and blood and urine samples collected regularly during the treatment period in order to quantify HIV-1 levels and determine body chemistries. Pharmacokinetic studies require additional blood sampling up to Week 16. [AS PER AMENDMENT 6/30/98: Pharmacokinetics data from Cohort I showed that the proposed Cohort II starting dose was too low. The dose for Cohort II is now increased. All subjects in Groups I, II, and III will begin combination therapy on Day 0 at the increased dose.] [AS PER AMENDMENT 3/13/00: The study has been extended for an additional 104 weeks, provided the patient's viral load is undetectable (below 400 copies/ml) at the end of the initial study period. While on the treatment extension, patients must continue their current schedule for study drug administration and completion of study visits.]