The possibility of grisin binding in animal organs and tissues was studied and more accurate methods for determination of grisin in animal foods were developed. Such products as meat, milk, fat, eggs and subproducts (liver, kidneys, lungs), as well as organs and tissues of rabbits were tested. The antibiotic was added to the feed in an amount of 400 mg per head. The animals and poultry were killed 1 hour after the feeding. It was shown that grisin was partially bound in vitro and in vivo in the organs and tissues and could not be detected with the routine methods. A procedure for enzymatic hydrolysis of the homogenates with pancreatin, a proteolytic enzyme was elaborated by the authors for determination of the adsorbed or bound antibiotic. The procedure of the enzymatic hydrolysis used alone or in combination with thermal treatment provided determination of 90-100 per cent of grisin contained in the products, while the routine method of homogenization without thermal treatment revealed only 50-60 per cent (or 20-35 per cent in the liver). The use of the procedure increased the accuracy of the grisin traces determination 2.8-15.6 times as compared to the routine methods. It allowed one to obtain more correct data on the true levels of the drug in organs and tissues.