We used [18F]-4-fluorobenzamido-N-ethylamino-maleimide ([18F]-FBEM) to radiolabel cells ex vivo for in vivo positron emission tomography (PET) in order to assess cell trafficking in mice. In contrast to commonly used imaging agents, [18F]-FBEM forms a covalent bond with thiol groups present on the cells surface. The stability of the probe in aqueous medium was tested at different pH values and cross-experiment showed that thiol-labeling efficiency was retained (at least) up to pH 9. The labeling procedure did not affect significantly the cell viability. To illustrate the procedure, PET images of living mice injected intravenously with labeled T lymphocytes were obtained. They showed the expected cell homing in the spleen that was absent in mice injected with free label. Copyright © 2013 John Wiley & Sons, Ltd.