We have investigated the effect of visible light on animal cells using light-emitting diodes to emit monochromatic visible light (red, yellow, green and blue light). To explore the relevant mechanism of apoptosis, we assessed the intracellular superoxide anion production and mitochondrial membrane potential (ΔΨm) of B16F1 and B16F10 murine melanoma cells after monochromatic light irradiation. Blue light caused ΔΨm depolarization subsequent to elevation of intracellular superoxide production. However, red and yellow light had no affect on both cell lines. Green light induced ΔΨm collapse only in B16F1 melanoma cells. ΔΨm is a key indicator of mitochondrial function, therefore its disruption causes mitochondria-dependent apoptosis. Thus, blue light causes mitochondrial dysfunction and subsequent cell death.