The human calcitonin (hCT) peptide hormone has a marked tendency to aggregate in aqueous solutions and to form long, thin fibrillar aggregates resulting in viscous and turbid dispersions. In this study, the in vitro aggregation products of hCT were systematically investigated using conventional transmission electron microscopy (CTEM) and in-lens field emission scanning electron microscopy. The mass per length of unstained/airdried specimens was determined by scanning transmission electron microscopy. Irrespective of the sample preparation method and electron microscopic (EM) imaging mode employed, similar supramolecular assemblies were observed. Based on these EM data, it is proposed that hCT aggregation beans with the formation of approximately 4-nm-thick protofibrils. These protofibrils further interact via lateral association and coiling to form higher-order fibrillar assemblies, i.e., protofibril-ribbons, fibrils, fibril-ribbons, tubes, and multi-stranded cables. The concentration and history of the aggregated hCT solutions strongly influenced the relative frequency of the various hCT assemblies depicted. The supramolecular assemblies of hCT revealed distinct helical symmetries at their different levels of aggregation. A hypothetical mechanism assumed for aggregating solutions to form polymorphic fibrillar hCT assemblies is presented in a schematic model, and the supramolecular arrangement of hCT within the various polymorphic fibrillar aggregates is delineated. (C) 1995 Academic Press, Inc.