The extracellular tumor microenvironment is acidified, whereas the intracellular pH of tumor and stromal cells is neutral. cis-Urocanic acid (cis-UCA), an endogenous compound of the skin, can acidify the cytosol by transporting protons into the cells. This phenomenon, termed the protodynamic concept, was studied here in human cancer cells. cis-UCA dose-dependently reduced the number of viable human melanoma, cervical carcinoma, and fibrosarcoma cells at weakly acidic extracellular pH. The intracellular pH decreased by up to 0.5 pH units in a concentration-dependent manner with 0.3-30 m cis-UCA at extracellular pH 6.5 but not at pH 7.4. Under the same conditions, 30 mM cis-UCA induced annexin-V binding and activation of caspase-3 in A2058 melanoma cells as signs of apoptotic cell death. Finally, growth of human melanoma xenografts in SCID mice was suppressed by 60% following intratumoral injection of cis-UCA. Accordingly, the percentage of tumor necrosis and active caspase-3-immunopositive cells increased, whereas proliferation activity decreased. These results identify cis-UCA as an anticancer agent inhibiting melanoma growth by immediate intracellular acidification followed by apoptotic cell death in vivo.