The contents of twelve minor and trace elements in liver, ascites, and blood of sarcoma-180 tumor-bearing mice were determined by instrumental neutron activation analysis at various tumor growing stages. Biological samples were irradiated by reactor neutrons and subsequently subject to direct analysis using a high-resolution HPGe -spectrometer. A statistically significant difference for the concentration of Na and Cl in all tumor samples relative to those in a normal control group is found. During the tumor growing period, concentration of Mg, K, Fe, Se, Rb, Cu, Zn, and Mn in liver and ascites of tumored mice vary with respect to the tumor size. Each elemental concentration and its biological function in the tumored mice is discussed and possible correlation to humans is analyzed.