The establishment of an experimental model with rabbits in which VX-2 carcinoma was implanted in the gastrocnemius muscle and subsequently successfully cured by a 2nd tumor cell inoculation was reported previously. Tumor growth and regression could be followed by manual palpation. The changes in serum ceruloplasmin (CP, EC 1.10.3.2) levels of individual rabbits during tumor development and regression were followed. CP levels increased 4- to 8-fold during progression, often before tumors could be detected by palpation. With tumor regression CP levels returned to normal. When metastases developed, the CP levels remained high. This phenomenon seems to be related to the VX-2 carcinoma, since CP levels in rabbits challenged with various antigens and suffering from induced multiple subcutaneous abscesses did not change significantly, while in pregnant rabbits CP levels increased up to at most 3-fold. Serum CP level can serve as a reliable biochemical marker of the activity of this malignant process. The practical application of this finding lies in the follow-up of malignant processes in humans and is now under investigation.