Activities to be carried under this contract:1) design and execute proficiency testing programs to externally assess the ability of U.S. and non-U.S. DAIDS-support clinical trial sites (hereinafter referred to as Sites) to reliably: 1) perform immunological assays (e.g., T-cell subsets and other cytometry-based tests); 2) provide assistance and training to laboratories that are not performing according to established standards, especially to flow cytometry laboratories in non-U.S. resource limited countries; 3) conduct or support comparative evaluations of instruments, methods and reagents; 4) facilitate the development, standardization and assay characterization of new immunological assays, and of cheaper simpler methods to measure CD4, for implementation in multi-center investigations; and 5) evaluate the ability of Sites to adequately freeze viable peripheral blood mononuclear cells (PBMC) for future immunological tests.