1. Division of Biomedical Radiation Science, Department of Diagnostic Radiology, Uppsala University, Box 535, S-751 21 Uppsala, Sweden; Pharmacia Biotech AB, S-751 82, Sweden; Department of Chemical Engineering, Mälardalen University, Box 325, S-63105 Eskilstuna, Sweden; and Biochemical Separation Centre, Uppsala Biomedical Centre, Box 577, S-751 23 Uppsala, Sweden