 Despite the emergence of high level drug resistance in HIV-infected patients on stable antiretroviral therapy, plasma HIV RNA levels generally remain below the pretherapy viral load set-point. The virologic and immunologic determinants of this lower steady state level of viremia have not been defined. Preliminary data indicate that: 1 drug resistant variants have reduced replicative capacity and pathogenic potential; 2 drug resistant viremia is associated with reduced T cell activation and turnover compared to wild-type viremia; and 3 patients with low level drug resistant viremia often have HIV-specific CD4 cells that are absent in patients with higher levels of viremia. This study will investigate whether the emergence of a poorly fit, drug resistant variant results in the generation of an effective HIV-specific CD4 cell response and if this response contributes to the establishment of a lower steady state level of viremia. 