The interaction between CD27 and CD70 provides a costimulatory signal for T-cell survival. Although the role of CD27 signaling in CD8⁺ T cells has been well defined, its role in CD4⁺ T cells is relatively unknown. Here, we report that CD70 is specifically expressed on differentiated T-helper (Th)1 cells, but not on Th2 cells. Upon activation, CD70 expression increased markedly on Th1 cells, but remained undetectable on Th2 cells. We demonstrate that CD27 is involved in naive T-cell expansion in Th1-type, but not in Th2-type, immune responses as in vivo treatment with anti-CD70 monoclonal antibody at induction resulted in a significant reduction of delayed-type and contact hypersensitivity responses, but not asthmatic responses. In both Th1-type responses, during the priming phase, CD70 was detected at earlier time points on dendritic cells (DCs) and at later time points on CD4⁺ T cells. Our results indicate that CD70 may be useful as a marker to distinguish Th1 from Th2 cells. More importantly, CD27 function may be controlled by the differentially regulated kinetics of CD70 expression on DCs and CD4⁺ T cells, and Th1 cell-specific CD70 expression may be involved in an amplification loop for polarized Th1-type immune responses through T cell–T cell interactions.