R ates of graft rejection are high among recipients of heart transplants. The onset and progression of clinically significant heart transplant rejection are currently monitored by serial biopsy, but this approach is highly invasive and lacks sensitivity. Here, we have developed what we believe to be a new technique to measure organ rejection noninvasively that involves the exploration of tissue-infiltrating leukocytes as biomarker sources for diagnostic imaging. Specifically, we profiled the myeloid response in a murine model of heart transplantation with the aim of defining and validating an imaging signature of graft rejection. Ly-6C^hi monocytes, which promote inflammation, accumulated progressively in allografts but only transiently in isografts. Ly-6C^lo monocytes, which help resolve inflammation, did not accumulate, although they composed the majority of the few remaining monocytes in isografts. The persistence of Ly-6C^hi monocytes in allografts prompted us to screen for a Ly-6C^hi monocyte–associated imaging marker. Low-density array data revealed that Ly-6C^hi monocytes express 10-fold higher levels of myeloperoxidase (MPO) than Ly-6C^lo monocytes. Noninvasive magnetic resonance imaging of MPO with an MPO-activatable Gd-chelate revealed a spatially defined T1-weighted signal in rejected allografts but not in isografts or MPO-deficient allograft recipients. Flow cytometry, enzymography, and histology validated the approach by mapping MPO activity to Ly-6C^hi monocytes and neutrophils. Thus, MPO imaging represents a potential alternative to the current invasive clinical standard by which transplants are monitored.