Hypoxia and low pH are commonly associated with the tumor microenvironment. We have examined the effects of hypoxia alone (HA) and hypoxia coupled to low pH (HApH) on p53 expression, nucleotide excision repair (NER) and cellular sensitivity to UVC in normal human fibroblasts and human tumor cells. p53 expression was measured using Western blotting, NER using host cell reactivation (HCR) of a UV-damaged reporter gene and cell sensitivity using the MTT assay. HApH resulted in a transient increase in p53 expression in normal fibroblasts at 6h and in tumor cells at 6-18h. In normal fibroblasts HApH resulted in a transient increase in HCR at early times (12-24h) and a concomitant decrease in UVC sensitivity. In contrast, for the tumor-derived cells, the increased HCR of the UVC-treated reporter gene was delayed (36-40h) and UVC sensitivity increased or remained the same after HApH treatment. These results suggest that early upregulation of p53 and increased repair of UV-damaged DNA after HApH treatment is required for increased cell viability after UVC. HA treatment alone also resulted in a transient increase in HCR of the UVC-damaged reporter gene at early times (12-24h) in normal fibroblasts and a delayed increase (36-40h) in the tumor-derived cells. However, the enhanced p53 expression was less or even absent for treatment with HA alone, and HA had no significant affect on cell viability after UVC for any of the cell lines. These results indicate a different cellular response following HApH compared to HA alone.