The expression of interleukin 8 (IL-8) by human ovarian cancer cells correlates directly with disease progression, but the exact mechanism of IL-8 induction is not clear. The extracellular pH in solid tumors is generally acidic because of elevated acid production and impaired clearance of acidic metabolic wastes. We determined whether acidic conditions also regulate the expression of IL-8 in human ovarian cancer cells. Culturing SKOV3 ip1 ovarian cancer cells in acidic medium (pH 6.6) significantly increased IL-8 mRNA (Northern blot) and protein(ELISA). The acidosis-mediated transient increase in IL-8 expression involved both transcriptional activation of the IL-8gene and enhanced stability of the IL-8 mRNA. Detailed functional analysis of the IL-8 promoter revealed that the sequence between −133 and −98 bp relative to the transcription initiation site was primarily responsible for IL-8 gene transcriptional activation by acidosis. Point-mutated luciferase reporter studies indicated that activator protein-1 (AP-1) and nuclear factor-κB(NF-κB)-like factor were responsible for acidic pH-induced transcriptional activation of the IL-8 gene, and EMSA demonstrated that both NF-κB and AP-1 bound to these sites on the IL-8 promoter. These results indicate that acidic pH activates NF-κB and AP-1 in human ovarian cancer cells and in doing so increases IL-8 gene expression.