Infection with the human immunodeficiency virus (HIV-1), the causative agent for the acquired immunodeficiency syndrome (AIDS) leads to a progressive decline in the number of CD4+ lymphocytes, ultimately resulting in a profound immunosuppression. With the introduction of new anti- retroviral and immunostimulatory therapies, it has become clear that the surrogate markers used at present, CD4 lymphocyte number and serum p24 antigen, are not sensitive enough to permit a rapid evaluation of new treatment modalities, particularly in the early stages of HIV infection. We have applied the technique of limiting dilution analysis (LDA) to HIV- infected population. The LDA is a sensitive and quantitative means for measuring immune competence, allowing the estimation of the frequency of lymphocytes responsive to a given antigen. Using this approach, we have shown that lymphocyte populations reactive with recall antigens such as tetanus toxoid are present and functional in HIV-infected persons at all stags of disease. However, we have also employed the LDA for detection of immunologic responses directed against immunodominant HIV envelope peptides. Frequencies of reactive lymphocytes, although readily detectable, are low in asymptomatic HIV-infected individuals. In this application, we propose to refine the LDA approach for measuring anti-HIV immune responses by investigating the following specific research questions: Can the sensitivity of the HIV-specific LDA be further increased? and What are the characteristics of the responding cell population in the HIV-specific LDA? Thereafter, specific clinically relevant research questions can be pursued. First, do HIV-specific immune responses play a role in protection from HIV infection by examination of couples where one sex partner is seropositive and the other seronegative. Second, do HIV-specific immune responses predict the course of HIV disease by performing LDA ama;uses retrospectively on those persons who progress versus those who remain asymptomatic. Third, do HIV-specific immune responses correlate with clinical efficacy in vaccine trials by measuring the anti-HIV immune response in those persons enrolled in ACTG protocol 137 (immunization of asymptomatic HIV-infected persons with gp160). From our results thus far, it is clear that the LDA provides an extremely sensitive approach for assessing immune competence and should be of value in evaluating the relevance of the HIV-specific cellular immune response as a predictor of disease progression or response to therapy.