The spin- and angle-resolved photoelectron spectroscopy from ultrathin Cr films on Fe(110) is investigated by means of first-principles electronic structure and photoemission calculations. The antiferromagnetic ordering in the Cr films lead in dependence on film thickness to a rapidly decreasing and oscillating photoelectron spin polarization, in reasonable agreement with recent experiments (Dedkov (2007) [1]). The oscillation period is explained by quantum-well states in the Cr film and by a Fermi surface nesting vector. The importance of transition matrix elements is highlighted. The findings point to a noncollinear magnetic structure at the Fe/Cr interface.