We have measured the magnetoresistance of high-vacuum-sputtered Co/Cu(100) multilayers grown on Cu buffer layers. The magnetoresistance in the first antiferromagnetic- (AF-) coupling peak is very sensitive to the buffer layer thickness. We find a linear dependence between the actual measured magnetoresistance and the fraction of AF coupling, as determined by magnetization measurements. We compare our Co/Cu(100) magnetoresistance data at 4 K of completely antiparallel-aligned multilayers with the quantum model of giant magnetoresistance of Levy, Zhang, and Fert. This reveals evidence for strong spin-dependent interface scattering, whereas the spin dependence of the bulk scattering in Co is small.