The effect of cycloprodigiosin hydrochloride (cPrG.HCl), a H+/Cl- symporter, on five human breast cancer cell lines (KPL-1, T-47D, MCF-7, MKL-F, and MDA-MB-231), a human breast epithelial cell line (HBL-100), and a human fibroblast cell line (WI-38-40) was examined. cPrG.HCl inhibited the growth of all five breast cancer cell lines (IC50: 0.46-0.62 microM) and slightly inhibited HBL-100 and WI-38-40 cell growth (IC50: 1.75 microM and 2.26 microM respectively). cPrG.HCl treatment in KPL-1 cells increased the pH of acidic organelles, decreased intracellular pH, and caused apoptosis, which was confirmed by the appearance of a sub-G1 population by flow cytometry and DNA fragmentation. In addition, cPrG.HCl-induced apoptosis was strongly suppressed by imidazole, a cell-permeable base, suggesting that intracellular acidification was essential for the apoptosis. Further, cPrG.HCl treatment up-regulated Bax and Bak expression, down-regulated Bcl-2 expression, and activated caspase-3. Therefore, the intracellular acidification by cPrG.HCl treatment suppressed the growth of human breast cancer cell lines by inducing apoptosis.