Flourine-19 (19F) MRI has demonstrated the ability to monitor immune cell populations in vivo. Murine green fluorescent protein natural killer (NK) cells were labeled with perfluoropolyether (PFPE-red) and injected intratumorally into lymphoma-bearing mice. In this work, we demonstrate the ability to track and quantify NK cells up to 5 days in vivo, via 19F MRI. In vivo quantification indicated that 88% of the initially detected NK cells remained at the tumor site on day 5. Flow cytometry analysis provided validation that the cellular label remains colocalized within a significant portion of the NK cells at day 5.