This report examines the use of 5‐(and‐6)‐carboxyfluorescein diacetate succinimidyl ester (CFSE) to determine the site, duration and cell type responsible for antigen presentation in vivo. Evidence that CFSE‐labelled T cells can be used to determine where various types of antigens are presented, including auto‐antigens, oral antigens and cell‐associated foreign antigens, is provided. Using this technique, the length of time antigen is presented after acquisition by APC was measured. Finally, CFSE labelling was used to identify the origin of the APC responsible for different forms of antigen presentation.