Some people's immune systems are able to control HIV infection without anti-HIV drugs. Other people with HIV must take drugs to prevent the virus from destroying their immune systems. There are many different laboratory tests that measure immune function in people with HIV. This study will compare some of these tests to see if they consistently measure differences between people who control the HIV without anti-HIV drugs and those who must take drugs.

The efficiency of the immune response to HIV antigens is the critical feature that allows some individuals with chronic HIV infection to maintain low level viremia (less than 3000 copies/ml). The fundamental measurement of this response is the steady state level of viremia in the absence of antiretroviral drugs. However, using this clinical endpoint in vaccine and drug trials is time-consuming. Several laboratory assays of HIV T cell function have been developed to measure the key characteristics of an efficient immune response. This study will evaluate these assays in two distinct patient populations.

Two patient cohorts will be followed in this study. Cohort A will enroll patients who are stable on highly active antiretroviral therapy (HAART). These patients will have been on the same HAART regimen for at least 9 months prior to study entry. Cohort B will enroll patients with chronic HIV infection and efficient immune control. These patients will have not been on any antiretroviral drugs for at least 6 months and will have viral loads less than 3,000 copies/ml. Participants in both cohorts will have blood drawn at study entry and Weeks 12 and 24. Blood samples will be used for CD4/CD8 cell count, plasma HIV-1 RNA, and immunologic assays.

Inclusion Criteria for Cohorts A and B:

• HIV-1 infection
• CD4 cell count > 300 cells/mm3 within 60 days prior to study entry
• Negative pregnancy test within 14 days of starting study
• Agree to use acceptable methods of contraception while in study

Inclusion Criteria for Cohort A (Stable HAART) Only:

• Stable HAART regimen, defined as the suppression of viral load to undetectable levels, for at least 9 months prior to study entry
• Viral load < 75 copies/ml on at least three occasions within 9 months prior to study entry, with at least one of these values obtained between 6 and 9 months prior to study entry
• No single viral load >= 75 copies/ml within 9 months prior to study entry

Inclusion Criteria for Cohort B (Efficient Immune Control) Only:

• Not taking any antiretroviral drugs for at least 6 months prior to study entry
• Meets study definition of efficient immune control (generally HIV-1 viral load < 3,000 copies/ml, with some exceptions)

Exclusion Criteria:

• Pregnancy or breast-feeding
• Active drug or alcohol use or dependence that, in the opinion of the investigator, would interfere with adherence to study requirements
• History of an AIDS-defining opportunistic infection