1. Norwegian Defence Research Establishment (FFI), P.O. Box 25, N-2027 Kjeller, Norway, Department of Bacteriology and Immunology, Norwegian Institute of Public Health, P.O. Box 4044, Nydalen, N-0403 Oslo, Norway, as Telelab, P.O. Box 1868, Gulset, N-3703 Skien, Norway, and Borregaard, P.O. Box 162, N-1701 Sarpsborg, Norway