We model the interaction between tumor cells and the immune system in the presence of a tumor growth modulator, either a promoter or inhibitor, by a set of differential equations. We make the following assumptions: 1. (1) In the absence of an immune response, tumor cells grow according to first-order kinetics (exponentially). 2. (2) A growth modulator, while it is present, either increases or decreases the proliferation rate of tumor cells. 3. (3) The part of the immune system that responds to the threat of tumor cells (we call them “defender cells”) destroys tumor cells according to a mass-action law. 4. (4) The presence of tumor cells stimulates the proliferation of cytotoxically active defender cells. However as the number of tumor cells increase beyond a certain threshold, the proliferation of defender cells is inhibited and the number of active defender cells diminishes. The model makes the following predictions: 1. (1) For promoters (modulators which increase tumor growth rate): There are two tumor-cell threshold levels, N 1 and N 2, which depend upon the strength of the promoter and the level of defender cells at the time they first engage the tumor. If the number of tumor cells at this time (the “initial tumor level”) exceeds N 2, the tumor will ultimately overwhelm the immune system, with or without the assistance of the promoter. If the initial tumor level falls between N 1 and N 2, the defender cells will destroy the tumor unless the promoter is permitted to act for a certain critical period of time, T̃. Once the promoter has acted for this critical time, it can be “turned off.” The tumor will overwhelm the defender cells nontheless. If the initial tumor level is below N 1, the defender cells will destroy the tumor even if the promoter is allowed to act indefinitely. 2. (2) For inhibitors (modulators which decrease the tumor growth rate): There are two threshold levels, N ' 1 and N ' 2, which depend upon the strength of the inhibitor and the level of defender cells at the time the inhibitor begins to act. If the number of tumor cells at this time (the “initial tumor level”) exceeds N ' 2, the tumor will ultimately overwhelm the immune system, no matter how long the inhibitor is allowed to act. If the initial tumor level lies between N ' 1 and N ' 2, the tumor will ultimately overwhelm the immune system unless the inhibitor is allowed to act for a certain critical time, T̃ '. Once the inhibitor has acted for this time, it can be turned off. The immune system will destroy the tumor. If the initial tumor level lies below N ' 1, the immune system will defeat the tumor, with or without the aid of the inhibitor. The model yields quantitative relationships between N 1, N 2, T̃ (or N ' 1, N ' 2, T̃ ' ) and the biological parameters of the system.