The inhibition of unspecific adhesion of human white blood cells is a prerequisite for applications requiring the control of defined surface interactions. In this study, a passivation agent based on polyethylene glycol (PEG) for glass surfaces was investigated for the use with human peripheral blood mononuclear cells (PBMC). The grafting of 2000 g/mol methoxy-terminated PEG-urea-triethoxysilane (mPEG2000) onto glass surfaces successfully inhibited unspecific spreading of both human PBMC and platelets in all experiments. The prevention of surface interactions was independent on the anticoagulant used during blood collection. The total efficiency to prevent even transient immobilization of PBMC to the PEG modified surfaces was 97 ± 2%. This makes the passivation with PEG a well suited surface modification for preventing unspecific surface interaction in order to study only defined surface interactions of human PBMC.