Abstract We have developed and optimized an in vitro functional assay using whole blood derived peripheral blood derived mononuclear cells (PBMC) from non-human primates. The assay was used to quantitate therapeutic specific T cell responses following dosing with a fully human antibody based therapeutic. Measles-specific response from same animals was used as a control. The drug product specific response was evaluated using 50 monkeys dosed with the therapeutic. The PBMC were tested for reactivity to the therapeutic and to a control measles peptide cocktail, using an IFN-gamma ELISPOT assay and a Luminex based cytokine secretion assay. The optimization and qualification experiments describe: i) evaluation of number of cells to be plated in culture, ii) pre-resting of cells before plating, iii) viability of cells, and iv) impact of co-stimulatory cytokines (IL2, IL7, and IL15) on elicitation of antigen-specific responses. An assay specific cut point and stimulation index was calculated to distinguish responders from non-responders. Using this qualified assay, therapeutic specific responses and measles-specific T cell responses were evaluated in 56 and 50 monkeys respectively. These assays may provide an assessment of antigen-specific T cell function in vivo, and have various applications, such as evaluation of immune competence status of animals, immunomodulatory effects of drug products, and understanding the T cell dependence of anti-drug-antibody immune responses.