Rak is a nuclear tyrosine kinase containing Src homology 2 and 3 domains at its NH2 terminus. We report here that the retinoblastoma tumor susceptibility gene product pRb associates with Rak in vivo and in vitro. Rak binds in the A/B pocket region of pRb, a region that is frequently mutated in human cancer, during the G1 and S phases of the cell cycle. Furthermore, Rak expression is elevated in G1, and transfection of Rak into NIH 3T3 cells results in a significant decrease in the number of emerging colonies. Thus, Rak is a tyrosine kinase with growth suppressing activity that may function, in part, through its interaction with pRb.