THE cellular protein p107 and the retinoblastoma protein (pRB) have many features in common. Most strikingly, they contain homologous protein domains that mediate interaction with the oncoproteins of several small DNA tumour viruses, including adenovirus El A and SV40 large-T antigen^1–9. In cells that do not contain these viral oncoproteins, pRB interacts with the cellular transcription factor E2F (refs 10–12) or a related protein termed DRTF1 (ref. 13). E2F associates with a form of pRB that is found primarily in Gl cells¹⁰. It seems that the E2F-pRB complex dissociates near the Gl-S boundary before the initiation of S phase, releasing free E2F and apparently stimulating the ability of E2F to activate transcription¹⁴. Cells that express E1A have no or little pRB-E2F complex, presumably because of the association of E1A with pRB^10–15. During S phase, E2F forms a second complex that contains cyclin A but apparently lacks pRB¹⁵. Here, we report that p107 is found in the cyclin A/E2F complex and that this complex also contains p33^cdk2. These observations suggest that p107 and pRB cooperate in the regulation of E2F activity, each affecting different stages of the cell cycle. Thus, by binding to pRB and pi 07, El A and large-T antigen target two distinct aspects of E2F regulation.