The ability of lonidamine [1-(2,4)-dichlorobenzyl-1H-indazol-3-carboxylic acid], to induce a stress response in human and murine cultured melanoma cells has been demonstrated. In the M14 and M10 human melanoma cell lines, lonidamine enhances the synthesis of a unique set of proteins, characterized by SDS-PAGE by an Mr of about 72 kDa. In the B16 murine melanoma cell line, exposure to lonidamine increases the synthetic rate of two polypeptides of mol mass 86 and 72 kDa, respectively. Lonidamine is a drug which specifically acts on mitochondria. Therefore the observation that it can also promote a stress response indicates that the mitochondria might be one of the primary cellular targets and postulates a causal relationship between an impairment of the energy supply and induction of stress protein synthesis.