Several cellular proteins form stable complexes with the proteins encoded by the adenovirus early region 1A (E1A) gene in extracts derived from adenovirus infected or transformed cells. Two of the cellular proteins that bind to E1A have been identified; one, a 105-kilodalton protein (pRb), is the product of the retinoblastoma gene, and the other, a 60-kilodalton protein, is a human cyclin A. Two other proteins that bind E1A have now been shown to be related to p34cdc2. This E1A complex displayed histone H1-specific kinase activity; the kinase activity was modulated during the cell division cycle, and association of pRb with E1A apparently was not required for this activity.