1. Cancer Research Campaign Departments of Carcinogenesis and Medical Oncology, Paterson Institute for Cancer Research, Christie Hospital NHS Trust, Manchester M20 9BX, U.K., Toxicology Division, TNO Nutrition and Food Research Institute, Utrechtseweg 48, P.O. Box 360, 3700 AJ Zeist, The Netherlands, and School of Epidemiology and Health Sciences, Medical School, University of Manchester, Oxford Road, Manchester M13 9PT, U.K.