The crosslinking theory of aging has been gaining acceptance at a steady pace, as evidenced by many independent rediscoveries. While several earlier studies were indicative, none seemed conclusive until it was shown, using Differential Scanning Calorimetry (DSC), that protein from young human brains could be made to closely resemble protein from old brains by exposing it to either of two entirely different crosslinking agents (glutaraldehyde and dipotassium diperoxy sulfate). This work has now been repeated with additional brain material, and a statistically more significant number of determinations. It is now shown that a treatment of brain protein with either one or two chemically totally different compounds which have no property in common except that both are crosslinkers, changes young brain protein so that it greatly resembles old, crosslinked protein. This shows that crosslinking reactions are involved in the age related changes in the studied proteins.