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By using an enzyme-bridge immunoperoxidase (PAP) technique, localization of so-called pregnancy-specific beta 1-glycoprotein (SP1) and placental-specific tissue proteins (PP5, PP10, PP11, PP12) was investigated in 19 cases of breast cancer, 24 cases of testicular malignant tumours, 12 cases of gastric cancer and some cases of other malignant tumours. These five glycoproteins isolated from human term placentae and characterized by Bohn are localised mainly in the cytoplasm, or nucleus of syncytiotrophoblast and appear to be specific for the trophoblast. But to a certain percentage these proteins could also be detected in the cytoplasma of malignant cells: In breast cancer, SP1 was present in 52.6% of cases, PP5 in 63.2%, PP10 in 68.4%, PP11 in 55.6% and PP12 in 31.6%. In testicular malignant tumours the detection rates of these proteins varied from 20.8 to 75.0% and in gastric cancer from 41.7 to 66.7%. In total 50% of the tumours tested were SP1-positive, 58.3% PP5-positive, 55.6% PP10-positive, 38.0% PP11-positive and 31.9% PP12-positive. In addition, it was found that mononuclear histiocytes showed a strong cytoplasmic staining for PP10 and PP12 and that a few other non-cancerous cells (i.e. striated cells in gastric metaplasia, gastric polyps etc.) stained for PP5, PP10, PP11 and/or PP12. All control sections were negative in malignant cells as well as in other tissue cells. This study confirms the reports that some kinds of malignant tumours produce SP1 and indicates that the ectopic production of proteins normally produced by the trophoblast is a common finding in malignant tumours. It furthermore suggests that such proteins may be useful as markers in monitoring patients with malignant diseases. The possible mechanisms of ectopic production of these inappropriate proteins in malignant tumours are discussed.