Adoptive transfer of chimeric antigen receptor (CAR)-T cells is expected to become the first line of treatment for multiple malignancies, following the enormous success of anti-CD19 therapies. However, their mechanism of action is not fully understood, and clear guidelines for the design of safe and efficient receptors are missing. We hereby describe a systematic analysis of the CAR “interactome” in human primary T cells, which allowed us to identify molecular traits that influence CAR-T cell efficacy. Interactome analysis was based on immunoprecipitation of CARs followed by protein identification by mass spectrometry.