Phenotypic revertants were isolated from simian virus 40-transformed cells in order to examine the relationship between simian virus 40 T-antigen expression and G1 arrest of growth. Revertant clones with increased adherence were selected from cultures of SVT2, a simian virus 40-transformed BALB/c mouse cell line, and screened to find arrestable revertant clones which inhibited DNA synthesis when crowded. The clones selected from untreated SVT2 were unstable and showed little or no inhibition of DNA synthesis when crowded. Stable revertants were found after treatment of SVT2 with Colcemid to increase ploidy. The stable revertants all lost most transformed growth properties tested, including tumorigenicity, but only a few showed the same degree of inhibition of DNA synthesis at high cell density as BALB/3T3. All revertant clones expressed T antigen at low cell density. Three revertants showed coordinate inhibition of DNA synthesis and apparent loss of T antigen at high cell density. We suggest that changes in gene dosage rather than mutations caused the altered properties of the new revertants and that continued DNA synthesis in confluent cultures may be the transformed phenotype that requires the least simian virus 40 T antigen.