The effects of photodynamic therapy (PDT) alone and in combination with 3(2)-t-butyl-4-hydroxyanisole (BHA) on Ehrlich ascites carcinoma (EAC) cells have been investigated. BHA, a widely used food antioxidant, administered to the cells prior to light exposure is found to cause concentration-dependent alterations of the haematoporphyrin derivative (HpD)-based PDT. BHA (0.15 mM) causes a small (about 10%) inhibition in the rate of HpD-photosensitized injury of EAC cells. In contrast, upon increasing the concentration of BHA from 0.15 to 0.5 mM, a 1.3-fold enhancement in HpD-PDT efficiency is achieved. The cytotoxic effect on the cells treated with HpD-PDT and a higher concentration of BHA (0.5 mM) is additive. When BHA (0.5 mM) is given immediately after HpD-PDT, the combination is found to be three to four times more effective than when BHA is added to EAC cells before phototherapy. In this treatment regimen BHA acts synergistically with HpD-PDT. Such a difference in the action of BHA on the efficiency of HpD-PDT might be explained by the ability of BHA to inhibit the HpD-photosensitized destruction of some biomolecules. An enhancing action of BHA on the intensity of HpD-photosensitized death of tumour cells is also observed in vivo. Even a single dose of BHA (0.6 mM kg-1, 15 min after irradiation) causes (in an additive manner) an approximately two-fold increase in the efficiency of HpD-PDT of mice bearing Ehrlich ascites tumour (intraperitoneal transplantation). The results obtained indicate that the potentiating effect of BHA on the HpD-PDT could be caused by the impairment of the mitochondrial respiration, since there is a good correspondence between the concentration of BHA that increases the efficiency of PDT and the concentration that inhibits the oxygen consumption and dehydrogenase activity of EAC cells. The influence of BHA on the efficiency of PDT does not depend on the nature of the photosensitizer used; the effects with chlorin-e6 trimethyl ester are similar to that seen for HpD.