On the basis of cell surface markers, mature T cells are considered to have either a naïve or a memory phenotype. These cells exhibit distinct types of kinetic behaviour in vivo. While naive‐phenotype cells persist long term in a non‐dividing state. memory phenotype T cells include cycling cells and exhibit a more rapid rate of turnover; this has also been shown to be true for cells that can be definitively identified as naive or memory T cells respectively. The number of memory‐phenotype (CD44in) CD8⁺ T cells entering cell cycle is greatly increased after In vivo exposure to viruses, bacteria or components of bacteria. Accelerated turnover of memory T cells also occurs after the injection of a variety cytokines that are induced by infectious agents, including type I interferon (IFN‐I), Although naive‐phenotype T cells do not divide in response to these cytokines, they do exhibit signs of activation, including upregulation of CD69 after exposure to lFN‐1, These findings suggest that the dissimilar in vivo kinetics of naive‐ and memory‐phenotype T cells might reflect their divergent responses to cytokines. Furthermore, the ability of infection‐ induced cytokines to stimulate non‐specific proliferation of memory phenotype T cells and partial activation of naive‐phenotype T cells implies that they play a complex role during primary immune responses w infectious agents.