The location of the high-affinity Mn2+ site in apo-photosystem (PS) II was investigated by pulsed EPR The electron electron magnetic dipole interaction of 1.7 MHz between the Y-D(center dot) radical and Mn2+ ion was observed using the pulsed electron electron double resonance (PELDOR) technique, and the Mn2+ ion was bound to one apo-PS II in the absence and presence of Ca2+. PELDOR signals were calculated using the previously determined spin distribution on the YD center dot radical and its known position in the crystal structure, assuming that the specific Mn2+ site was located in the oxygen evolving complex. The results show that the high-affinity Mn2+ site is located at the position denoted by Mn4(A) in the native crystal structure. The Mn2+ is coordinated with axial ligands Asp170 and Glu333 in the D1 polypeptide.