Heavy metals have been implicated in the aggregation of proteins and the pathophysiology of several neurodegenerative diseases. Herein, we describe the interaction of recombinant human factor VIII (rhFVIII) with Al ΓΎ3 , Tb ΓΎ3 , Co ΓΎ2 , and Fe ΓΎ3 using a combination of intrinsic fluorescence, circular dichroism, and highresolution fourth-derivative absorbance analysis. rhFVIII in solution was titrated with the metal cations and the properties of the resulting complexes were examined. rhFVIII has a tendency to aggregate and inactivate slowly over time under physiological conditions, but this aggregation process is greatly accelerated in the presence of metals with Al ΓΎ3 being the most efficient. This leads to a complete loss of activity of the protein. Al ΓΎ3 -induced conformational changes in the protein were small but detectable with limited changes seen in secondary and tertiary structure. Because rhFVIII is a multidomain protein with subunits linked through divalent metal cations, the small intramolecular changes seen may be attributed to rearrangements of the subunits to an aggregation-competent conformer that is very similar to that of the native form. Γ 2004