Infection with the human immunodeficiency virus results in profound perturbations in immunological memory, ultimately resulting in increased susceptibility to opportunistic infections and acquired immunodeficiency syndrome (AIDS). W e have used rhesus macaques infected with the simian immunodeficiency virus (SIV) as a model to understand better the effects of AIDS virus infection on immunological memory. Acute infection with SIV resulted in significant deficits in CD4+ helper responses to cytomegalovirus (CMV) as well as CMV–specific cytotoxic T–lymphocyte and neutralizing antibody responses. Reactivation of CMV was associated with high levels of SIV replication and suppression of both T–helper and cytotoxic responses to CMV . We have also studied the effects of SIV infection on T–cell turnover in non–human primates. T–cell turnover was evaluated using the nucleoside analogue bromodeoxyuridine (BrdU) in combination with five–colour flow cytometric analysis. T cells in normal animals turned over at relatively rapid rates, with memory cells turning over more quickly than naive cells. In SIV–infected animals, the labelling and elimination rates of both CD4+ and CD8+ BrdU–labelled cells were increased by two– to threefold compared with normal controls. Further analysis of immunological memory in nonhuman primates should offer the opportunity to extend immunological insights from murine models to the pathogenesis and prevention of AIDS.