CD26 inhibition confers a competitive engraftment advantage on donors. An in utero competition analysis was performed to mechanistically evaluate the effect of CD26 inhibition on allogeneic donor BM cells used for IUHCT. A total of 10 × 10⁶ CD26-inhibited B6GFP BM cells were coinjected with 10 × 10⁶ noninhibited congenic B6Pep3b cells into 14-day–gestation BALB/c fetuses. The contribution to peripheral-blood engraftment of the 2 donor-cell populations was assessed postnatally and is expressed as the percentage of CD45⁺ cells that are from the inhibited and noninhibited donor; n = 5. *P < .05 when comparing chimerism levels between the 2 donor-cell populations.