The toxicity of the methylxanthine derivative A802715 is determined against the two human melanoma lines, Be11 and MeWo, and against the two human squamous cell carcinoma lines, 4197 and 4451, by vital dye staining assay. A802715, with a TD50 of 0.9-1.1 mM, is the most toxic. BrdU incorporations in p53 wt cells show that A802715 strongly enhances the irradiation-induced suppression of S-phase entry. Depending on the p53 status of the cell line, A802715 either prolongs the G2/M block or remains ineffective[1].