This study is designed to determine whether patients previously primed with Ad5 of MRKAd5 HIV-1 gag vaccine respond better when boosted with ALVAC-HIV vaccine than when boosted with MKRAd5 HIV-1 gag vaccine.

Inclusion Criteria:

• Subject was previously enrolled in Merck HIV Vaccine study V520 Protocol 007 or 012, and received the complete 3-dose regimen during participation
• Subject is in good general health
• Subject is not infected with HIV
• Subject agrees not to donate blood during the first 52 weeks of the study
• Subject agrees not to donate sperm during the first 12 weeks of the study
• Subject who is of reproductive potential agrees to use an acceptable method of birth control through Week 12 of the study

Exclusion Criteria:

• Subject has been administered immune globulin or blood product 3 months prior to receiving study vaccination
• Subject has been vaccinated with a live virus vaccine in the past 30 days
• Subject has been vaccinated with an inactivated vaccine in the past 14 days
• Subject has an active medical disease
• Subject is taking daily required prescription drugs
• Female subject is pregnant, breastfeeding or expecting to conceive
• Subject is positive for HIV
• Subject has used injection drug within the past year
• Subject has a sexual partner that is infected with HIV
• Subject has a sexual partner that is an active injection drug user
• Subject has been treated for or diagnosed with a new sexually transmitted disease in the past 6 months
• Subject engaged in unprotected intercourse with more than 2 persons in the previous 6 months
• Subject engaged in protected intercourse with more than 4 persons in the previous 6 months
• Subject has previously participated in an HIV vaccine clinical trial (other than Merck V520 Protocols 007 and 012)
• Subject weighs less than 110 lbs.
• Subject has a recent history of alcohol abuse
• Subject intends to donate blood in the first 52 weeks of the trial