Loss of tumor suppressors contributes to numerous cancer types. Many, but not all, proteins encoded by tumor suppressor genes have antiproliferative activity and halt cell-cycle progression. In this chapter, we present three methods that have been utilized to monitor the antimitogenic action exerted by tumor suppressors. Tumor suppressor function can be demonstrated by colony formation assays and acquisition of the flat-cell phenotype. Because of the anti-proliferative action of these agents, we also present two transient assays that monitor the effect of tumor suppressors on cell-cycle progression. One is based on BrdU incorporation (i.e., DNA replication) and the other on flow cytometry. Together, this triad of techniques is sufficient to determine the action of tumor suppressors and other antiproliferative agents.