Little is known about the S100B+ lymphocytes, which are unique human peripheral blood lymphocytes (PBL) containing the S100B protein. It has recently been shown that S100B is released from various types of S100B+ cells and exhibits varied cytokine-like activities. In this study, we precisely characterized the S100B+ lymphocytes of healthy adults with respect to the proportion in the whole PBL, immunophenotypes, function, and their S100B mRNA expression and also evaluated their S100B-releasing activity upon stimulation. S100B+ lymphocytes were detected in all individuals examined, and the proportion of S100B+ lymphocytes in the whole PBL ranged from 0.42% to 16.15% (mean, 4.21%). In addition, two subtypes of S100B+ lymphocytes, a CTL subtype (CD3+ CD8+ CD16-) and a NK subtype (CD3- CD8- CD16+), were detected. The majority of the CTL subtype of S100B+ lymphocytes expressed the αβ-T-cell receptor. Surprisingly, S100B mRNA was detected not only in S100B+ lymphocytes, but also in every S100B- lymphocytes, although the expression levels of S100B mRNA in S100B- lymphocytes were much lower than those of S100B+ lymphocytes. The CTL subtype of S100B+ lymphocytes exhibited blastic morphological changes, proliferated and released S100B upon stimulation with phytohemagglutinin. The NK subtype of S100B+ lymphocytes exhibited morphological NK activity when cocultivated with NK-sensitive target, K-562 cells. Thus, the CTL subtype of S100B+ lymphocytes exhibit the biological characteristics of T cells, while the NK subtype of S100B+ lymphocytes exhibit the characteristics of NK cells. These results suggest that S100B+ lymphocytes are a particular subtype of cytotoxic lymphocytes that play a unique role in antitumor immunity.