Semisynthetic [3H]insulin was stored under various conditions for up to 180 days and the stability of the insulin under these conditions was assessed. A sample that had been stored for 180 days was repurified and shown to be degraded at the same rate as native insulin by perfused mouse livers, even at low physiological concentrations. After perfusion, intact insulin could be separated from degradation products, and the radioactivity associated with the insulin fraction could be used to determine the percentage degradation. The initial rate of degradation of insulin was a linear function of concentration over the range 360pM-1.9nM.