Ochratoxin A (OTA)-methyl ester was the main metabolite produced after ochratoxin A exposure. It is known, that OTA shows potent renal toxicity and carcinogenicity in rodents, with a tendency to develop a renal tumor. Recent experiments revealed that the hybrid Caco-2/HepG2 co-culture system might be a promising tool for assessing the cytotoxicity parameters and the biotransformation of OTA. Both cells Caco-2 and HepG2 metabolized OTA to OTA methyl ester at the concentrations where this mycotoxin shows not cytotoxic effect. In this sense, the use of the methylated metabolite of OTA reported as the major metabolite could be a novel biomarker of OTA in biological samples.