PURPOSE. Mature-onset cataract results from the formation of light-scattering aggregates of lens crystallins. Though oxidative or mutational damage may be a prerequisite, little is known of the initiation or nucleation of these aggregated states. In mice carrying mutations in gamma-crystallin genes, a truncated form of gamma-crystallin formed intranuclear filamentous inclusions within lens fiber cells. In this work we investigated amyloid fibril formation of human gammaC-crystallin (HgammaC-Crys) induced by low pH, together with characterization of a partially unfolded intermediate in the process. METHODS. The presence of partially unfolded intermediates were detected by tryptophan fluorescence spectroscopy and UV resonance Raman spectroscopy. The aggregation into amyloid fibrils was monitored by solution turbidity and ThioflavinT fluorescence assay. The morphology of aggregates was characterized using transmission electron microscopy. Secondary structure of the peptides in their fibrilar state was characterized using FTIR. RESULTS. The structure of HgammaC-Crys was perturbed at low pH. Partially unfolded intermediates were detected when solution pH was lowered to pH 3. At pH 3, HgammaC-Crys aggregated into amyloid fibrils. The kinetics and extent of the reaction was dependent on protein concentration, pH, and temperature. TEM images of aggregates revealed aggregation stages from short to long fibrils and from long fibrils to light-scattering fibril networks. FTIR spectroscopy confirmed the cross-beta character of the secondary structure of these fibrils. CONCLUSIONS. HgammaC-Crys formed amyloid fibrils upon incubation at low pH via a partially unfolded intermediate. This process could contribute to the early stages of the formation of light scattering species in the eye lens.