The evaluation of antigen-specific T-cell responses is helpful for both research and clinical settings. Several techniques can enumerate antigen-responsive T cells or measure their products, but they require remarkable amounts of peripheral blood mononuclear cells (PBMCs). Since screening numerous antigens or testing samples from pediatric or lymphopenic patients is hampered in clinical practice, we refined a miniaturized, high-throughput assay for T-cell immunity. Antigens and cells in 10-l volumes were dispensed into 1,536-well culture plates precoated with anti-gamma interferon (anti-IFN-␥) antibodies. After being cultured, the wells were developed by enzyme-linked immunosorbent assay for bound cytokine. Miniaturization and automation allowed quantitation of antigen-specific responses on 10 4 PBMCs. This method was applied for epitope mapping of mycobacterial antigens and was used in the clinic to evaluate T-cell immunity to relevant opportunistic pathogens by using small blood samples. A comparison with conventional methods showed similar sensitivity. Therefore, current flow cytometric methods that provide information on frequency and phenotype of specific T cells can be complemented by this assay that provides extensive information on cytokine concentrations and profiles and requires 20-to 50-fold fewer PBMCs than other analytical methods.Numerous methods are currently available to enumerate antigen-specific T cells and to measure their functions, most of them being based on flow cytometric analysis. Such methods have been compared and critically discussed in recent reviews (4, 17). A common feature of these methods is the requirement, on average, of 2 ϫ 10 5 to 5 ϫ 10 5 peripheral blood mononuclear cells (PBMCs) to test a single antigen. This imposes a limitation on the analytical antigenic breadth, which depends on the number of available PBMCs. In the case of lymphopenic subjects, the population that most frequently needs to be monitored for immunocompetence, this is particularly crucial. Also, the small volumes of pediatric blood samples may result in inadequate numbers of PBMCs for extended analysis of specific T-cell immunity. By the same token, screening of large peptide panels for epitope mapping on PBMCs from healthy donors (blood donors or vaccinees) is also limited to a few hundred peptides (16) instead of the thousands of peptides needed to cover several immunodominant proteins (14). Therefore, we felt the need to develop a miniaturized assay that makes use of limited numbers of PBMCs, thereby enhancing our screening power.A miniaturized assay based on 384-well plates instead of conventional 96-well plates or 5-ml tubes was developed in our laboratory and named cell-enzyme-linked immunosorbent assay (cell-ELISA) (8). This method was based on a previous observation that lymphocyte cultures can be run in 96-well plates in which the wells had been precoated with an anticytokine antibody. The cytokine secreted by antigen-specific T cells was captured by the solid-phase antibody, and the plate was de...