Fluorine-19 (19F) MRI can monitor various cell types in vivo for days to weeks after injection of 19F-labeled cells. To establish cell viability Green Fluorescent Protein (GFP+) murine natural killer (NK) cells were labeled ex vivo using red-fluorescent perfluoropolyether (PFPE-red) and delivered intratumorally into 3 lymphoma-bearing mice. Flow cytometry validated that the infused NK cells retain their 19F label out to 6 days post-injection (PI). Cells were then tracked and quantified via 1H/19F MRI out to 6 days PI. Quantification of the infused cells indicate 87% and 70% were detectible at days 0 and 6, respectively.