The present invention relates to a method for investigating body fluids for cancer cells, the use thereof and corresponding analysis kits, and the possibilities for cancer treatment derived therefrom. The method is based essentially on determining the expression of the manganese superoxide dismutase, thioredoxin reductase and/or glutathione peroxidase genes. Use of this method permits in particular reliable tumor diagnosis and prognosis. Diminishing an elevated expression of these genes has therapeutic value and may be utilized for cancer treatment.