We have examined the ability of Ad5 ElA 12S viruses with deletions in ElA exon 1 to induce quiescent baby rat kidney cells to progress through the cell cycle and to undergo mitosis. Measurements of mitotic index and analyses by fluorescence activated cell sorting were correlated with the abilities of the mutant ElA proteins to bind to cellular proteins. All the mutants induced cells to leave GO/G1 and enter S phase, but two groups were defective at inducing mitosis, and cells infected with them appeared to be blocked between the S and M phases. The first group of mutants, with deletions in the regions of residues 4–25 and 30–60, bound p300 poorly or not at all and gave reduced numbers of mitoses. The second group, with deletions between residues 111 and 138 in CR2, failed to bind pRb and were completely defective at inducing mitosis. In this group, mutants lacking residues between 124 and 138 bound p107 and cyclin A at much reduced levels and induced cells to overreplicate their DNA. The site in ElA required to bind cyclin A extends from residue 124 to at least 127. Cyclin A binds to a 107-kDa cellular protein, which by peptide analysis appears identical to pl07.