Group C: ALVAC placebo and alum placebo. [AS PER AMENDMENT 10/12/00: ALVAC vCP1452 and AIDSVAX B/B at Months 0, 1, and 6 and ALVAC vCP1452 and AIDSVAX placebo at Month 3.] Group D: ALVAC vCP1452 and alum placebo at Months 0 and 1 and ALVAC vCP1452 and MN rgp120 at Months 3 and 6. [AS PER AMENDMENT 10/12/00: ALVAC placebo and AIDSVAX placebo at Months 0, 1, 3, and 6.] Group E: ALVAC vCP1452 and AIDSVAX B/B at Months 0, 1, and 6; ALVAC vCP1452 and alum placebo at Month 3. [AS PER AMENDMENT 10/12/00: Group E has been discontinued.] Group F: ALVAC vCP1452 and AIDSVAX B/B. [AS PER AMENDMENT 10/12/00: Group F has been discontinued.] Group G: ALVAC vCP1452 and alum placebo at Months 0 and 1; ALVAC vCP1452 and AIDSVAX B/E at Months 3 and 6. [AS PER AMENDMENT 10/12/00: Group G has been discontinued.] Following each pair of injections, volunteers are observed in the clinic for 30 minutes. Volunteers keep a record of body temperature and any other symptoms and report results to their AIDS Vaccine Evaluation Unit. Evaluations are performed by telephone or clinic visit on Days 1 and 2 after each vaccination. [AS PER AMENDMENT 10/12/00: Volunteers record all relevant signs and symptoms occurring 48 hours after each vaccination and provide that information at each clinic visit.] HIV testing is conducted every 3 to 6 months and volunteers are asked to complete social harms questionnaires once at Day 168 and once at the end of the study. Volunteers are followed on the study for a minimum of 18 months after the first immunization. Safety is evaluated by closely monitoring for local and systemic adverse reactions during the course of the trial.