The memory T cell response is polyclonal, with the magnitude and specificity of the response controlled in part by the burst size of T cells expanded from effector/memory precursors. Sensitive assays using HLA class II multimers were used to detect low-frequency Ag-specific T cells directed against influenza viral Ags in subjects immunized with the influenza vaccine. Direct ex vivo tetramer staining of PBMC from five individuals identified frequencies of hemagglutinin (HA) 306–318 tetramer binding CD4+ T cells in the peripheral blood ranging from 1 in 600 to 1 in 30,000 CD4+ T cells. These frequencies were validated by counting CFSE low, tetramer-positive T cells after in vitro expansion. Low frequency of T cells directed to other influenza epitopes, including DRA1* 0101/DRB1* 0401-restricted matrix protein 60–73, DRA1* 0101/DRB1* 0101-restricted matrix protein 18–29, DRA1* 0101/DRB1* 0701-restricted HA 232–244 and DRA1* 0101/DRB1* 0101-restricted nucleoprotein 206–217 were also determined. T cells which occurred at a frequency as low as 1 in 350,000 could be ascertained by in vitro expansion of precursors. Peripheral HA 306–318-responsive T cells expanded 2-to 5-fold following influenza vaccination. Examination of phenotypic markers of the HA 306–318-responsive T cells in the peripheral blood indicated that the majority were CD45RA−, CD27+, CD25−, CD28+, and CD62L−, while T cell clones derived from this population were CD45RA−, CD27−, CD25+, CD28+, and CD62L−.