The authors experimentally reproduced in dogs, sclerosing lesions of biliary tract similar to those that they noticed in clinical practice, after the use of a formalin solution for sterilization of the hydatid cysts of the liver. These lesions have been found with formalin solutions more concentrated than those used in surgical treatment of liver echinococcosis. Nevertheless, even with the use of lower concentrated solution in every case, inflammatory scars are produced in the liver and the biliary tract with persist several months after the formalin injection consequently, the authors recommend to forsake the abandonment of this product for surgical treatment of hydatid cyst.