A series of experiments is carried out to identify the contribution of interface and bulk antiferromagnetic (AFM) spins to exchange bias (EB) in ultrathin epitaxial ferromagnetic (FM)/AFM bilayer samples. These are single-crystalline AFM Ni𝑥Mn100−𝑥 and ferromagnetic Co layers on Cu3Au(001), deposited under ultrahigh vacuum conditions, in which structural or chemical defects are deliberately introduced by controlled Ar ion sputtering at the surface of the AFM layer or at a certain depth inside the AFM layer. Comparison of the magnetic properties measured by magneto-optical Kerr effect for sputtered and nonsputtered parts of the same sample then allows a precise determination of the influence of sputtering on the AFM layer during the sample preparation, whereas all other parameters are kept identical. The results show that the creation of defects in the bulk of the AFM layer enhances the magnitude of EB and its blocking temperature, but not the creation of defects at the interface. It is also observed that the deeper the insertion of defects in the AFM layer, the higher the value of the EB field and the larger the coercivity, These findings are discussed as the effect of additional pinning centers in the bulk of the AFM layer.