Two different human mammary carcinoma cell lines were xenotransplanted into nude mice. Serum samples were obtained prior to and after transplantation and investigated by two-dimensional electrophoresis (2-DE). By comparison of these silver-stained patterns additional protein spots were detected resulting either from proteins secreted or shed by the tumor itself or from mouse proteins induced by the tumor or the transplantation procedure. One group of spots detectable in post-transplantation serum as well as in control serum after mock-transplantation but not in pretransplantation serum was microsequenced and identified as mouse P-haptoglobin. The carbohydrate structures of P-haptoglobin were characterized by two different immunochemical glycoprotein, staining procedures to detect differential terminal glycan modifications.