The protofibril monoclonal antibody PFA1 recognizes multiple Aβ species. 5 μL of serum (S) and 10 μg of soluble protein extract from brains (B) of 22 month old AD 3 × tg mice were immunoblotted for Aβ with antibody PFA1 (left) or 6E10 (right). 1 μM synthetic Aβ40 and Aβ42 (right lanes) were loaded for comparison. Membranes were cut at 15 kDa to subject the lower panels to a longer exposure for detection purposes. PFA1 detected the 56 kDa oligomeric form of Aβ from serum with high affinity, similar to 6E10.