The stability of the (W.H.O.) 4th International Standard for Insulin, has been assessed by accelerated thermal degradation studies. This is a crystalline preparation of insulin, freed from proteolytic enzymes, sealed in ampoules containing air and with a moisture content of 5--6%. Of the original biological activity 95.8 (92.8--98.9;P = 0.95)% was retained after storage for 12 years in the dark at 20 degrees C and 65.7 (63.4--68.1;P = 0.95)% after 14 years at 37 degrees C. Degradation rate constants were calculated from these data for the Standard at 20 degrees C and 37 degrees C, assuming first order kinetics. The degradation constant at 37 degrees C did not differ significantly from those found in earlier degradation studies at 37 degrees C over shorter periods, thereby supporting the assumption that the degradation of crystalline insulin, at least at 37 degrees C, is a first order reaction. Extrapolation of these data suggest that the Standard stored at -20 degrees C for 20 years would have retained at least 99.93% (P = 0.95) of its original activity and so for practical purposes can be considered to be stable.
