In vitro stability of 99mTc-radiopharmaceuticals is an important parameter of quality. In the paper the shelf-life of the bone-seeking radiopharmaceutical 99mTc(Sn)-pyrophosphate was examined under different experimental conditions.The solution of pyrophosphate was prepared and dispensed into 2-ml fractions which were kept at-20 degrees C. In different time intervals the samples were thawed and labeled. The in vitro stability of labeled preparation was examined on the unprotected and the samples protected by nitrogen purge or by addition of solutions of ascorbic or genetisic acid of known concentrations. The content of 99mTc-pertechnetate was determined by paper chromatography.In comparison with unprotected, samples protected by nitrogen purge exhibit better, but not satisfactory stability. Best results are obtained by using chemical stabilizers. The presence of 50-60 micrograms/ml of ascorbic acid keeps the content of 99mTc-pertechnetate below 5% during six hours even in solutions of high initial radioactive concentrations (up to about 814 MBq/ml). With this concentration of ascorbic acid, good results are obtained also by labeling inactive solutions kept at low temperatures during seven days. Genetisic acid is also efficient but its concentration should be about ten times higher.The in vitro stability of 99mTc(Sn)-pyrophosphate depends on the initial radioactive concentration of 99mTc and time after labeling. Best results were obtained when the preparation contained ascorbic acid as a chemical stabilizer. Its concentration in the range of 50-60 micrograms/ml is sufficient to keep the content of 99mTc-pertechnetate below 5% during six hours, both in freshly prepared samples and those kept at low temperatures for seven days.