The Chemoprevention Branch of the Division of Cancer Prevention and Control at the National Caner Institute directs a program to identify and develop drugs for human use that will prevent, delay, or reverse the progression of cellular dysplasia (e.g. intraepithelial neoplasia) to invasive cancer. part of the drug research and development process involves preclinical toxicology studies. These studies are performed under Good Laboratory Practices according to guidance documents published by the Food and Drug Administration and are used to support Investigational New Drug applications and eventually New Drug Applications to the marketing approval of cancer chemopreventive agents. The contractor shall conduct a 13-week toxicity study of 1,4- Phenylenebis(methylene) selenocyanate in dogs. In a previous study of this compound thrombocytopenia was observed. This study is designed to evaluate the mechanism of this effect and to test the hypothesis that it is based on a drug-induced immune response.