The ubiquitously expressed c-Abl tyrosine kinase is localized to the nucleus and binds to DNA. The DNA binding activity is regulated by cdc2-mediated phosphorylation, suggesting a cell cycle function for c-Abl. Here we show that the tyrosine kinase activity of nuclear c-Abl is regulated in the cell cycle through a specific interaction with the retinoblastoma protein (RB). A domain in the C-terminus of RB, outside of the A/B pocket, binds to the ATP-binding lobe of the c-Abl tyrosine kinase, resulting in kinase inhibition. The RB-c-Abl interaction is not affected by the viral oncoproteins that bind to RB. Hyperphosphorylation of RB correlates with release of c-Abl and activation of the tyrosine kinase in S phase cells. The nuclear c-Abl tyrosine kinase can enhance transcription, and this activity is inhibited by RB. Nuclear c-Abl is an S phase-activated tyrosine kinase that may participate directly in the regulation of transcription.