Cyclin E is classified as a putative G1 cyclin on the basis of its cyclic pattern of mRNA expression, with maximal levels being detected near the G1/S boundary. We report here that cyclin E is found associated with the transcription factor E2F in a temporally regulated fashion. E2F is known to be a critical transcription factor for the expression of some S phase-specific proteins and is thought to be important for a series of others. Antisera specific for cyclin E were raised and used to demonstrate an association between cyclin E and E2F. This cyclin E/E2F complex was seen in a variety of human cell lines from various tissues, but its appearance was detected primarily during the G1 phase of the cell cycle. The cyclin E/E2F association decreased as cells entered S phase, just as the association of E2F with cyclin A became detectable. We characterized the cyclin E-E2F complex further to show that both the cyclin-dependent kinase-2 (cdk2) and p107 were present. Therefore, the p107/E2F complex is associated with two different cdk2 kinase complexes--one containing cyclin A and the other containing cyclin E--and the appearance of these complexes is temporally regulated during the cell cycle. The presence of cyclin E/E2F complexes in the G1 phase suggests a role for cyclin E in the control of genes required for the G1-to-S transition.