Serum-stimulation of quiescent mouse ®broblasts results in transcriptional activation of tissue factor (TF), the cellular initiator of blood coagulation. This requires the rapid entry of c-Fos into speci®c AP-1 DNA-binding complexes and can be strongly inhibited by the adenovirus E1A 12S gene product. In this study, we utilized a panel of E1A mutants de®cient in cellular protein binding to analyse the molecular basis for E1A inhibition of a minimal, c-Fos-dependent TF promoter/ reporter construct in mouse AKR-2B ®broblasts. Mutations which impaired binding of the retinoblastoma tumor suppressor protein family members pRB, p107, and p130 relieved E1A-mediated inhibition of transcription in response to serum-stimulation or c-Fos overexpression. Inhibition was restricted to the G 0 to G 1 transition, consistent with the speci®city of E1A for hypophosphorylated forms of RB proteins. Although E1A mutants de®cient in CBP/p300 binding retained the ability to inhibit TF transcription, deletion of the aminoterminal portion of the CBP/p300 interaction domain was required to permit rescue of TF promoter activity by coexpression of pRB. Moreover, ectopic p107 could e ectively substitute for pRB in relieving E1A-mediated repression. In primary mouse embryo ®broblasts, activity of the minimal AP-1-dependent TF promoter was suppressed in Rb 7/7 cells compared to parallel Rb +/7 and Rb +/+ transfectants. Ectopic expression of either pRB or p107 markedly enhanced TF promoter activity in Rb 7/7 ®broblasts. Collectively, these data imply that pRB and p107 can cooperate with c-Fos to activate TF gene transcription in ®broblasts and suggest a requirement for another, as yet unidenti®ed, E1A-binding protein. Oncogene (2000) 19, 3352 ± 3362.