Abstract: It is an object of the invention to provide methods and compositions for characterizing particles in a size range from about 1 ?m to about 10 nm, and preferably 5 ?m to about 5 nm. Using transmission electron microscopy and digital image processing techniques, the methods of the present invention can provide detailed information on the aggregation state of, for example, proteinaceous samples such as antibody-based pharmaceutical compositions. The methods can further permit assessment of the effect of storage, use, processing, and shipping conditions in such proteinaceous samples.