The murine Unp gene encodes a widely expressed ubiquitin-specific protease. The predicted sequence of the UNP protein features motifs common to viral oncoproteins through which these proteins interact with the retinoblastoma gene product pRb, as well as the related 'pocket proteins' p107 and p130. We have explored the possibility that UNP interacts with pocket proteins, and report here that such associations can be detected in vitro and in cells. Associations of UNP and pocket proteins are sensitive to site-directed mutations in a manner directly analogous to those documented in viral oncoproteins. We conclude that within cells UNP does physically associate with pRb, and can also associate with p107 and p130.