A comparative in vitro study between the effects of Ukrain (a new synthetic thiophosphoric acid derivative of great celandine alkaloids) and alkylating antitumor drugs cyclophosphamid and cisplatin on total thiol content in Guerin carcinoma, Guerin/cis-DDP carcinoma, and in animal livers was carried out. It is shown that Ukrain action on thiol groups in Guerin carcinoma tissue does not differ from that of cyclophospamid and cisplatin to both of which Guerin carcinoma is very sensitive. Once tumor resistance to cisplatin has developed, cisplatin does not react with tumor thiol groups and cyclophosphamid reactions with tumor thiol groups decrease. Reactions of Ukrain with thiol groups of cisplatin resistant tumors increase. This indirectly indicates the increase of tumor sensitivity to this drug. Therefore, the cytotoxicity of Ukrain is similar to that of known antitumor drugs and can probably overcome the cisplatin resistance of the tumor.