To determine whether the elevation of serum haptoglobin (Hp) elicited by many tumors is associated with properties of the tumor, Hp levels were determined during successive passages of two transplantable fibrosarcomas and two leukemia lines in syngeneic mice. Characteristic and unique profiles were elicited by each of the four tumors and were reproducible in each of three successive transplant generations. The Hp profiles elicited by EL4 leukemia cells were similar in allogeneic and syngeneic mice, except that the Hp maxima were greater in the allogeneic mice. Preimmunization with EL4 cells or pretreatment with immune serum or spleen cells obliterated the Hp response normally elicited by EL4 cells in allogeneic mice. These results suggest that the Hp response elicited by a tumor is associated with transmissible characteristics of the tumor.