Abstract: The invention is directed to a detection reagent for CAR expressing cells according to general formula (1) Xn—Sm—Do wherein X is a polypeptide comprising at least 20 amino acids of which at least two amino acids are connected by at least one disulfide group S is a spacer comprising at least 15 carbon atoms D is a detection moiety m, o are integers independently between 1 and 200 n is an integer between 4 and 200 with the provisio that X binds to the antigen binding domain of a chimeric antigen receptor of a cell. The detection agent can be used in a method to detect cells having an antigen binding domain of a chimeric antigen receptor (CAR) binding the detection reagent and preferable to remove the detection moiety D from the detected cell.