Previously, we showed that Src tyrosine kinases are activated early in the development of human colon cancer and are suppressed as intestinal cells differentiate. We identified RACK1 as an endogenous substrate, binding partner and inhibitor of Src. Here we show (by overexpressing RACK1, depleting Src or RACK1 and utilizing cell-permeable peptides that perturb RACK1's interaction with Src) that RACK1 regulates growth of colon cells by suppressing Src activity at G(1) and mitotic checkpoints, and consequently delaying cell cycle progression. Activated Src rescues RACK1-inhibited growth of HT-29 cells. Conversely, inhibiting Src abolishes growth promoted by RACK1 depletion in normal cells. Two potential mechanisms whereby RACK1 regulates mitotic exit are identified: suppression of Src-mediated Sam68 phosphorylation and maintenance of the cyclin-dependent kinase (CDK) 1-cyclin B complex in an active state. Our results reveal novel mechanisms of cell cycle control in G(1) and mitosis of colon cells. The significance of this work lies in the discovery of a mechanism by which the growth of colon cancer cells can be slowed, by RACK1 suppression of an oncogenic kinase at critical cell cycle checkpoints. Small molecules that mimic RACK1 function may provide a powerful new approach to the treatment of colon cancer.