The studies were undertaken to evaluate the effect of long-therm storage and methods of preservation (freezing and storage at -12 and -30°C and freeze-drying) on the suitability of bovine thymus as a raw material for the pharmaceutical industry. Quantity changes in soluble protein, amino nitrogen and pH were determined. The greatest changes in soluble protein were observed for freeze-dried samples, while for amino nitrogen for gland stored at -12°C. pH changes were insignificant in all analysed methods of storage. Investigations indicated the best stability of the gland stored at -30°C.