Most human cancers show perturbation of growth regulation mediated by the tumour-suppressor proteins retinoblastoma (RB) and p53 , indicating that loss of both pathways is necessary for tumour development. Loss of RB function leads to abnormal proliferation related to the deregulation of the E2F transcription factors, but also results in the activation of p53, which suppresses cell growth. Here we show that E2F-1 directly activates expression of the human tumour-suppressor protein p14^ARF (the mouse homologue is called p19^ARF), which binds to the MDM2-p53 complex and prevents p53 degradation,. These results complete a pathway linking abnormal proliferative signals, such as loss of RB, with the activation of a p53 response, through E2F-1 and p14^ARF. They suggest that E2F-1, a protein inherently activated by cell-cycle progression, is part of a fail-safe mechanism to protect against aberrant cell growth.