Isolated antibodies have been characterized which show specific affinity to a repeating conformational epitope of a protofibril form of the human ²-amyloid peptide as compare to low molecular weight forms of ²-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 ²-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.
