The in vitro stability of99mTc (Sn)-PyP as a function of experimental conditions of the preparation of the kit and time elapsed after labeling has been tested. The preparation was protected by using nitrogen-purged reactant solutions and kit vials and by ascorbic acid. The samples under nitrogen are stable for 6 h when the content of99mTc-pertechnetate raises to 5%. The best stability was achieved by addition of 5 g of ascorbic acid per ml of the kit (content of99mTc-pertechnetate about 0.5%). To accelerate the decomposition, exogenous hydrogen peroxide was used. In this case it was found that the presence of 10 g of ascorbic acid inhibits the effect both of oxygen and peroxide (6 g H2O2/ml of the kit). Radiochemical purity of99mTc (Sn)-PyP remains practically unchanged for 6 h (content of99mTc-pertechnetate about 0.5%).