CD4+ Tregs specific for noninherited maternal antigens (NIMAd) are detectable in some but not all B6 × BDF1 backcross, H‐2b homozygous offspring, and their presence is strongly correlated with extent of maternal (BDF1) microchimerism. We hypothesized that the level of pretransplant donor antigen‐specific Tregs could predict allograft tolerance. To test this idea, mice were screened for bystander suppression in a DTH assay, followed 1 week later by DBA/2 heterotopic heart transplantation. NIMAd‐exposed, H‐2b offspring that failed to suppress DTH uniformly rejected heart allografts (12/12) by d15. In contrast, 5/6 NIMAd‐exposed DTH ‘regulators’ accepted their allografts >100 days. The defect in ‘nonregulator“ offspring could be corrected by transfer of CD4+CD25+, but not CD4+CD25neg or CD8+T cells from transplant acceptor mice. In conclusion, donor‐specific T reg screening of F1 backcross offspring correctly predicted which recipients would accept a heart allograft. If translated to the clinic, similar pretransplant Treg screening could greatly enhance the effectiveness of tolerance as a clinical strategy in transplantation between family members.