1. Department of Veterinary Pathology, Veterinary Faculty, University of Utrecht, Yalelaan 1, PO Box 80.158, 3508 TD de Uithof-Utrecht, The Netherlands
2. Central Institute for the Breeding of Laboratory Animals TNO, PO Box 167, 3700 AD Zeist, The Netherlands
3. Royal College of Surgeons of England, 35-43 Lincoln's Inn Fields, London, WC2A 3PN, United Kingdom