The relationship between cell pigmentation and radiosensitivity was investigated in a cell model in which melanogenesis was suppressed by a glycosylation inhibitor. It was found that X-irradiation of melanotic B-16 melanoma cells and their amelanotic counterparts, obtained by glucosamine treatment, showed an inverse correlation between radiosensitivity and melanin contents. Since melanogenesis interruption by glucosamine does not affect the DNA repair capacity of nonpigmented cells, it is likely that intracellular melanins play a role in the relative resistance of pigmented cells to X-irradiation.