A mathematical model developed by Hege and Cole for evaluating antibody response was applied to estimate total tumor cell numbers within an animal’s body. The present model was useful only for the exponential phase of tumor growth; the tumor cell population could be quantitated and its growth related to the growth of the mouse. At the terminal stage of the disease, the tumor mass approximated 5–6% of the weight of the animal. © 1974 Oxford University Press. All rights reserved.