The Human Immunodeficiency Virus (HIV) is the etiologic agent of acquired immunodeficiency syndrome. The gag protein p24 (MW 24kD) is the structural protein of the HIV capsid. There are approximately 2000 p24 molecules per HIV particle. During acute HIV infection, virus replicates exponentially, and p24 becomes detectable in blood. Prior to host antibody response to the virus, detectable p24 strongly correlates to detectable viral RNA. Following seroconversion, host antibodies form complexes with p24, complicating p24measurement by immunoassay.