Conventional T1- or T2-MRI contrast agents do not allow to track the distribution of different cell populations simultaneously because the effects of relaxation enhancers are additive. Herein, it is shown that paramagnetic chemical exchange saturation transfer agents offer the opportunity to visualize different cell populations in vitro and in vivo by 1H-MRI. Yb- and Eu-HPDO3A complexes have been used to label murine macrophages (J774.A1) and melanoma cells (B16-F10), respectively. By selective irradiation of the highly-shifted OH resonances of the two chemical exchange saturation transfer agents, it has been shown that tracking of the two cell types is possible. These PARAmagnetic Chemical Exchange Saturation Transfer agents have a tremendous potential for clinical translation as they share the same stability and in vivo pharmacokinetic properties of Gd-HPDO3A (ProHance®), which is a widely used clinically approved MRI agent. Magn Reson Med, 2013. © 2012 Wiley Periodicals, Inc.