The influence of physical and chemical treatments upon the immunologi-cally effective subcellular fractions (5, 000×g supernatant and 105, 000×g sediment), which were prepared from the spleen cells of Yoshida sarcoma (YS)-resistant Donryu rats, was studied. The immunological activity of the 5, 000×g supernatant was stable to heating at 80°C for 30 min. It was stable to alkali (pH 10) and less stable to acid (pH 2). It was labile to a ten-day storage at 4°C, but relatively stable to a 30-day storage at -20°C. When the 105, 000×g sedi-ment was lyophilized and stored at -20°C for 95 days, its immunological activity was well maintained. It was labile to 95% ethanol, 90% phenol and 2 M NaNO₂, but relatively stable to 10% as well as 100% acetone, 10% phenol and 10% ethanol. It was labile to 0.1 M NaIO₄ and relatively stable to 0.1M K₂Cr₂O₇ solution. It was labile to RNase but relatively stable to DNase.