Abstract Adoptive therapy with chimeric antigen receptor (CAR) T cells shows great promise clinically. However, there are important aspects of CAR-T-cell biology that have not been explored, particularly with respect to the kinetics of activation, immune synapse formation, and tumor cell killing. Moreover, the effects of signaling via the endogenous T-cell receptor (TCR) or CAR on killing kinetics are unclear. To address these issues, we developed a novel transgenic mouse (designated CAR.OT-I), in which CD8+ T cells coexpressed the clonogenic OT-I TCR, recognizing the H-2Kb–presented ovalbumin peptide SIINFEKL, and an scFv specific for human HER2. Primed CAR.OT-I T cells were mixed with SIINFEKL-pulsed or HER2-expressing tumor cells and visualized in real-time using time-lapse microscopy. We found that engagement via CAR or TCR did not affect cell death kinetics, except that the time from degranulation to CAR-T-cell detachment was faster when CAR was engaged. We showed, for the first time, that individual CAR.OT-I cells can kill multiple tumor cells (“serial killing”), irrespective of the mode of recognition. At low effector:target ratios, the tumor cell killing rate was similar via TCR or CAR ligation over the first 20 hours of coincubation. However, from 20 to 50 hours, tumor cell death mediated through CAR became attenuated due to CAR downregulation throughout the time course. Our study provides important insights into CAR-T–tumor cell interactions, with implications for single- or dual receptor–focused T-cell therapy. Cancer Immunol Res; 3(5); 483–94. ©2015 AACR. See related commentary by June, p. 470