Isotypes of CD45 have been used extensively as markers of memory and naive populations of T cells in peripheral blood. In this study, T cells were isolated from human tonsil and their proliferative response against human rhinovirus was measured. Unexpectedly, equivalent responses were found among the CD4 + CD45RA + and CD4 + CD45RO + populations of T cells. This response requires MHC class II-positive antigen-presenting cells. The time course of the T cell response in vitro was that of a classical recall response, and no proliferative response to the virus could be detected in human cord blood. These results suggest that tonsils contain a significant population of CD45RA + memory cells. The presence of this population may reflect ongoing stimulation with this common infectious agent, and the anatomical location of the T cells within the major lymphoid organ draining the naso-pharyngeal epi-thelial surface.