The levels of cytokeratins (CK) in serum of cancer patients have been widely used for monitoring progression of cancer growth and the effectiveness of cancer treatment. Previous studies have shown that the release of CK by tumors in patients is a complex process which depends on the rate of cell damage caused by an increasing tumor mass, or by the tumor treatment, but is not in any simple manner correlated to the number of proliferating cells or to the total tumor mass (1). The complexity of the CK-releasing process has been analyzed by a computer model which mimics the progress of tumor growth, allows the introduction of different types of treatment (i.e. irradiation, chemotherapy and surgery), and computes the amount of CK released by the tumor, and the level of CK in blood and blood clearance. The computer model can be used to obtain a better understanding of the interactions of various factors, for scheduling of treatment and CK sampling, and for analyzing the effects of treatment.