Lentigo maligna and malignant melanoma can be treated by dicarboxylic acids (C9 and C12), which are competitive inhibitors of tyrosinase. We therefore studied the intracellular location and possible sites of action of dodecanedioic acid (C12) in murine melanoma cells, using EM autoradiography and biochemical analysis of lipid extracts by HPLC. Significant levels of radioactivity were found in the mitochondria and in the nuclei but not in association with membranes of rough endoplasmic reticulum, Golgi-associated endoplasmic reticulum, or Golgi apparatus, and not in coated vesicles or melanosomes. Biochemical analysis revealed that the diacid underwent beta-oxidation, which occurs only in mitochondria. The results suggest that the toxicity of dicarboxylic acids in melanoma cells is not related to anti-tyrosinase activity but may be due to interference with oxidoreductase enzymes in the mitochondria and possibly to inhibition of DNA synthesis in the nucleus.