In an attempt to identify protein-protein interactions with the Sp1 transcription factor, we have used a modified yeast `two-hybrid' system to clone cDNAs encoding proteins that interact with both the retinoblastoma-susceptibility gene product (RB) and the ubiquitous transcription factor Sp1. Using the two-hybrid screening assay we have isolated and sequenced novel cDNA species that likely encode for proteins that directly regulate or contribute to the regulation of Sp1 by the Rb gene product. Our previous examination of the cyclin homologue of the human uracil-DNA glycosylase (UDG2) reveal tight and cell cycle-specific regulation by the Rb gene product through Sp1 -binding elements of the human UDG2 gene. UDG2 is suspect in the supression of tumors based on its chromosomal localization and has biochemical activity that inhibits cyclinE/cdk2 kinase activity. The timing of Rb -mediated regulation of Sp1 coincide with phosphorylation of Rb by cdk 2 and cdc2 kinases. Interestingly, some of the cDNAs we have cloned may represent proteins important for regulating the phosphorylation of both Sp1 and Rb. We believe this mechanism, contribute to the diverse and pleotropic actions rendered by Rb which may be used ultimately to control cell growth in response to cell growth and/or DNA damage through transcription mediated by Sp1.