A need exists for biomarkers that can monitor transplant rejection. Now, Vallabhajosyula et al. show that, in a human-to-mouse xenogeneic islet transplant model, human exosomes (extracellular vesicles that are released by tissues) can be purified from the plasma of recipient mice. Moreover, islet rejection was associated with a marked reduction in the number of human exosomes. Importantly, donor kidney-specific exosomes were detected in five patients who underwent renal transplantation. The researchers propose that exosomes have potential as biomarkers of transplant rejection.