To study if photochemical treatment of human plasma by methylene blue in combination with visible light induces protein alterations, we analysed by high-resolution two-dimensional gel electrophoresis (2-D PAGE) untreated and photochemically treated fresh frozen plasma collected by apheresis from the same donor (n = 8). Polypeptides were separated according to their charges by isoelectric focusing and to their sizes in polyacrylamide gels in presence of sodium and to their sizes in polyacrylamide gels in presence of sodium dodecyl sulphate, and visualized by silver staining. Despite their apparent complexity, protein maps of untreated plasma and photochemically treated fresh frozen plasma revealed identical spot patterns. Thus, photochemical treatment did not induce apparent protein pattern modifications.