1. Division of Occupational and Environmental Medicine and Psychiatric Epidemiology, University Hospital, 221 85 Lund, Sweden, Division of Investigative Pathology, 1901 South First Street, Building 205, room 1R33, Temple, Texas 76504, and Biological Engineering Division and Department of Chemistry, Massachusetts Institute of Technology, 77 Massachusetts Avenue, room 56-731, Cambridge, Massachusetts 02139