1. Istituto Superiore di Sanita', Laboratory of Comparative Toxicology and Ecotoxicology, Viale Regina Elena 299 - 00161 Rome Italy, ChemQuest, 19-21 Cheyney St., Steeple Morden, UK, Centre for Molecular Design, University of Portsmouth, U.K., and Pharmacia and Upjohn, Kalamazoo, Michigan 49007-4949