In eukaryotes, transcription factors of the E2F family, in addition to having a role in cell proliferation, participate in regulating apoptosis, differentiation and development. In Arabidopsis thaliana, eight gene se- quences have been identified as encoding E2F or DP homologues. DP proteins form heterodimers with E2Fs. The aim of this work was to characterize the functions of three of these factors: AtE2F-a, AtE2F-b and AtDP-a. Here we report that AtE2F-a and AtE2F-b transactivate a reporter gene via an E2F consensus cis-acting element in Arabidopsis protoplasts. AtE2F-a is a more potent activator than AtE2F-b. Furthermore, co-expression of the E2F partner AtDP-a, or the DNA binding protein AtPurm, modulates the activation of AtE2F-a. Taken together, these results suggest that AtE2F-a, AtE2F-b and AtDP-a share features characteristic of members of the E2F family of transcription factors. Moreover, over- expression of AtE2F-a and AtDP-a can induce differ- entiated, non-dividing, leaf cells to re-enter S-phase. We conclude that the transcription factor AtE2F-a plays an important role in progression into S phase, which probably correlates with its capacity to stimulate transcription.