Each day 14,000 people become infected with HIV/AIDS, making the development of an effective vaccine one of the world’s top public health priorities. David Watkins’ laboratory is attempting to develop HIV vaccines that elicit cellular immune responses utilizing the simian immunodeficiency virus (SIV) – infected rhesus macaque animal model. A major component of the cell-mediated immune response are cytotoxic T-lymphocytes (CTL). It is thought that CTL play an important role in controlling HIV and SIV. Most standard immunological assays do not measure antiviral activity directly, limiting our understanding of CTL effectiveness. To address this, the Watkins laboratory developed a novel neutralization assay that quantifies the ability of virus-specific CTL populations to control viral growth. Evaluating the antiviral activity of CTL of different specificities will identify those CTL most effective against SIV. This information will likely impact the design of future HIV vaccines.