Memory Thy-1 + CD8 + T cells specific for the influenza A virus nucleoprotein (NP 366-374 ) peptide were sorted after staining with the D b NP 366 tetramer, labeled with CFSE, and transferred into normal Thy-1.2 + recipients. The donor D b NP 366 + T cells recovered 2 days later from the spleens of the Thy-1.2 + hosts showed the CD62L low CD44 high CD69 low phenotype, characteristic of the population analyzed before transfer, and were present at frequencies equivalent to those detected previously in mice primed once by a single exposure to an influenza A virus. Analysis of CFSE-staining profiles established that resting tetramer + T cells divided slowly over the next 30 days, while the numbers in the spleen decreased about 3-fold. Intranasal infection shortly after cell transfer with a noncross-reactive influenza B virus induced some of the donor D b NP 366 + T cells to cycle, but there was no increase in the total number of transferred cells. By contrast, comparable challenge with an influenza A virus caused substantial clonal expansion, and loss of the CFSE label. Unexpectedly, the recruitment of naive Thy-1.2 + CD8 + D b NP 366 + host D b NP 366 + T cells following influenza A challenge was not obviously diminished by the presence of the memory Thy-1.1 + CD8 + D b NP 366 + donor D b NP 366 + set. Furthermore, the splenic response to an epitope (D b PA 224 ) derived from the influenza acid polymerase (PA 224-233 ) was significantly enhanced in the mice given the donor D b NP 366 + memory population. These experiments indicate that an apparent recall response may be comprised of both naive and memory CD8 + T cells.