P1154 Aims: Primates offer valuable models for translational tolerance induction studies. However, molecular reagents mechanistically useful to study cell trafficking and chimerism are currently limited in that species. Here we evaluate a method well established for human chimerism studies to identify DNA unique to either recipient or donor among pairs of MHC-mismatched monkeys. Methods: Human short tandem repeat (STR) sequences were amplified by standard PCR using monkey DNA as template to define informative primer sets. Ten cynomolgus monkeys from various sources were analyzed. Informative STR primers were then validated using monkey DNA mixtures and then used to quantify recipient DNA within the graft during acute rejection, chronic rejection, and in the absence of rejection in eight monkey heart allografts, based on the ratio of donor to recipient STR products in PCR Results: Six STRs were analyzed including 3 in non-coding and 3 in coding DNA sequences. No amplicons were generated from monkey DNA using human non-coding sequences. However, amplicons similar in size to human alleles were observed for STRs in human genes FMR1, IT15 and TP53. Since TP53 was the most polymorphic locus analyzed, it was used in subsequent studies. DNA mixture experiments confirmed the specificity of the method, and defined the sensitivity limit at about 1%. The amount of recipient DNA in stable non-rejecting grafts ranged as high as 21%, whereas a normal graft within one day after transplant was below detection, as expected. Recipient DNA comprised a high proportion of total DNA in both acute (51 +/- 6%) and chronic rejection (47 +/- 6%), a result which correlated with parallel HE analysis of infiltrate density. Conclusions: This is the first report that human STR markers are suitable for quantifying macrochimerism in cynomolgus monkeys. This method could be used to detect small (>1%) populations of recipient cells infiltrating a graft, or to quantify migration of donor cells within recipient immune compartments. However, approaches that are more sensitive will be necessary to detect very low levels of mixed chimerism. This technique is complementary to antibody-based strategies to allotype, isolate, and further phenotype cell populations relevant to tolerance induction protocols in monkeys.