DNA damage results in an increase in P53 levels, which is required to initiate a P53-mediated cell cycle arrest and/or apoptosis. P53 and MDM-2 form a feedback control loop: while P53 can transactivate the MDM-2 gene, high levels of MDM-2 inhibit P53 transactivation as well as promote rapid degradation of P53. In the present study, we investigated the interaction between endogenous MDM-2 and P53 following UV-induced DNA damage in an MDM-2 overexpression cell line. A human osteosarcoma cell line (OsACL, which contains wild-type P53 and overexpresses MDM-2 protein) was used in this study. Here we show that following UV treatment, P53 levels increased in the OsACL cells despite the presence of high-level endogenous MDM-2; however, CAT assays using a P53 reporter system revealed that this P53 was transcriptionally inactive. Although P53 transactivation was inhibited, MDM-2 levels rose markedly following UV irradiation. Northern blot analysis revealed that the increase in MDM-2 protein levels was a result of increased levels of MDM-2 mRNA, possibly due to increased transcription. Cell cycle analysis revealed that OsACL cells were markedly resistant to UV-induced apoptosis. Transfection of OsACL cells with an anti-sense MDM-2 plasmid dowregulated MDM-2 expression and increased UV-induced apoptosis. In conclusion, MDM-2 overexpression can block UV-induced cell cycle arrest and apoptosis by inhibiting P53 transcriptional activity. Furthermore, increased expression of MDM-2 in OsACL cells following UV irradiation appears to be related to P53-independent mechanisms.