For detection of plasma proteins produced by human malignant tumors, a survey of blood plasma obtained from nude mice bearing serially transplanted human tumors was performed by immunoelectrophoresis and the double immunodiffusion technique. Among 34 lines including 18 types of human tumors, human specific plasma proteins were demonstrated in the plasma of nude mice transplanted with two lines of renal cell carcinoma, one adenocarcinoma of the colon, and one squamous cell carcinoma of the maxillary sinus. These tumors can be designated as "ectopic" plasma protein-producing tumors since the organs or tissues from which they originated are not considered to be usual sites of plasma protein synthesis. Plasma protein production, as well as that of alpha1-fetoprotein, was also found in one line of hepatoblasotma and three lines of yolk sac tumors. The above tumors were shown to produce one or more of the following 10 of 20 plasma proteins examined: albumin, prealbumin, alpha1-antitrypsin, ceruloplasmin, alpha2-macroglobulin, hemopexin, haptoglobin, C3 and C4 component of complement, and transferrin. An immunochemical demonstration of human specific cancer products observed in human tumors xenotransplanted into nude mice may provide a new approach for investigating the metabolism of neoplastic cells.