The aim of the present study was to evaluate the effects of ursodeoxycholic (UDCA) and deoxycholic (DCA) acids on the viability and proliferation of cultured virus-transformed tumour cells. The following permanent cell lines were used as model systems in our study: LSCC-SF-Mc29 (transplantable chicken hepatoma induced by the myelocytomatosis virus Mc29) and LSR-SF-SR (transplantable sarcoma in a rat induced by Rous sarcoma virus strain Schmidt-Ruppin). The investigations were performed by MTT test, neutral red uptake cytotoxicity assay and colony-forming method. The compounds tested were applied at a concentration range of 10-200 μg/ml for 24, 48 and 72 h. The results reveal that UDCA and DCA decrease significantly the viability and proliferation of the treated cells in a time- and concentration-dependent manner.