Formation of amyloid-like fibrils by insulin was studied at different insulin concentrations, pH and temperatures. At low pH (pH 2.5) the insulin fibrillization occurred only at high ([10 lM) peptide concentrations, whereas at physiological pH values the fibril formation is inhibited at higher insulin concentrations. The enthalpy of activation Ea of the fibril growth at pH 2.5 equals to 33 kJ/mol, which is considerably lower than 84 kJ/mol at physiological pH. The fibrillization rate of insulin decreases with increasing pH at high, 250 lM concentration, which was opposite to the pH effect observed in 2.5 lM insulin solutions. The latter effect indicates that protonation of histidine residues seems to be important for the fibrillization of monomeric insulin, whereas the pH effect at high concentration may result from off-pathway oligomerization propensity. Together, the different effect of environmental factors on the insulin fibrillization suggest that the reaction rate is controlled by different molecular events in acidic conditions and at physiological pH values.