Lonidamine (LND) is an antispermatogenic and antitumour agent acting via inhibition of the energy metabolism. According to our results LND in vitro acted as a photosensitizer enhancing synergistically the lethal action of UV radiation (lambda max = 330 nm, the range between 260-390 nm) towards Ehrlich carcinoma cells (EAC). The primary targets of phototoxic action of LND probably were cell membranes and mitochondria. UV irradiation of EAC in the presence of LND increased the permeability of the plasma membranes, stimulated the photoperoxidation of lipids, enhanced the inhibition of dehydrogenase activity and oxygen consumption of the cells. Deficiency of oxygen substantially decreased phototoxicity of LND. LND may induce photosensitized destruction of biomolecules by acting through type 1 and 2 reactions. It could be supposed that negative side effects of LND (e.g., photophobia and photosensitivity that have been reported for some cancer patients treated with LND) could be associated with its photosensitizing properties.