The concentration of iron (III)-transferrin (IT) in whole blood and serum, along with another high-spin (five unpaired electrons) iron complex (probably IT) accumulated by tumor tissue, was investigated by electron paramagnetic resonance (EPR) spectroscopy during the development of Murphy-Sturm rat lymphosarcoma. The observed changes in concentration (microgram/ml) of IT in sera/blood were generally complementary to those from tissue and the character of the modifications suggested the existence of three distinct phases of systemic response to the implantation: (1) an initial response, evidenced by a sharp reduction in serum IT and somewhat high tissue-IT concentration (microgram/g); (2) a period in which the tumor is (2) a period in which the tumor is becoming established, indicated by relatively constant tissue IT levels and near normal serum IT; and (3) the onset of rapid cell multiplication, characterized by increased total tissue-IT accumulation that rises to above 200% of normal available serum iron, increasing tissue-IT concentration, and rapidly declining serum-IT concentration. The results suggest that, in the face of an implanted tumor there are two detectable abnormal serum-IT responses: (1) an initial change, probably due to systemic blockage of iron reutilization; and (2) extraction of IT from serum by multiplying tumor cells, which is probably a major contributor to reduced serum-IT levels and ultimately anemia.