Activated haptens useful for generating immunogen to HIV protease inhibitors, immunogens useful for producing antibodies to HIV protease inhibitors, and antibodies and labeled conjugates useful in immunoassays for the HIV protease inhibitor saquinavir. The novel haptens feature an activated functionality at the central, non-terminal hydroxyl group. Also described are monoclonal antibodies specific for saquinavir having less than 10% cross-reactivity with lopinavir, nelfinavir, amprenavir, ritonavir, and indinavir, and a murine hybridoma producing said antibodies.