Near-infrared Raman spectroscopy may provide a highly sensitive, noninvasive means to identify activation status of leukocytes. The purpose of the current study was to establish Raman spectroscopic characteristics of T cell activation. Activation of the RsL.11 T cell clone in vitro with Con A resulted in specific decrements in band intensities at 785, 1048, 1093, and 1376 cm(-1) but did not alter a majority of other band intensities including those at 1004 cm(-1) (phenylalanine) and 1660 cm(-1) (amide bonds). Activation-dependent decrements in these band intensities occurred subsequent to IL-2 production and correlated closely with T cell blastogenesis. Activation-dependent decrements in these band intensities were not strictly a function of cell size because the same observations were noted in size-controlled comparisons of resting and activated T cells. Like the RsL.11 clone, freshly isolated thymocytes that were activated by Con A or IL-2 showed decrements in particular emissions. These findings indicate that near-infrared Raman spectroscopy can be used as a noninvasive technique to reveal the activation status of single living T cells.