Under conditions relevant to the manufacturing of insulin (e. g., pH 3, room temperature), biosynthetic human insulin (BHI), and Lispro insulin (Lispro) require a nucleation step to initiate aggregation. However, upon seeding with preformed aggregates, both insulins rapidly aggregate into nonnative fibrils. Far ultraviolet circular dichroism (far-UV CD) and second derivative Fourier transform infrared (2D-FTIR) spectroscopic analyses show that the fibrillation process involves a change in protein secondary structure from alpha-helical in native insulin to predominantly beta-sheet in the nonnative fibrils. After seeding, Lispro aggregates faster than BHI, likely because of a reduced propensity to reversibly self-associate. Composition gradient multi-angle light scattering (CG-MALS) analyses show that Lispro is more monomeric than BHI, whereas their conformational stabilities measured by denaturant-induced unfolding are statistically indistinguishable. For both BHI and Lispro, as the protein concentration increases, the apparent first-order rate constant for soluble protein loss decreases. To explain these phenomena, we propose an aggregation model that assumes fibril growth through monomer addition with competitive inhibition by insulin dimers. Biotechnol. Bioeng. 2011; 108: 2359-2370. (C) 2011 Wiley Periodicals, Inc.