<p>Isolated antibodies have been characterized which show specific affinity to a repeating conformational epitope of a protofibril form of the human ß-amyloid peptide as compare to low molecular weight forms of ß-amyloid peptide. These isolated antibodies and related pharmaceutically effective compositions may be useful in the therapeutic and/or prophylactic treatment of Alzheimer's disease by effectively blocking the ability of the protofibril form of ß-amyloid peptide to form fibril forms linked with complications associated with Alzheimer's disease. The isolated antibodies of the present invention are also useful in various diagnostic assays and associated kits.</p>