Abstract: To study the influence of decay time on the results of bacterial endotoxin test, the content of bacterial endotoxins in technetium ^99mTc methylenebisphosphonate injection (^99mTc-MDP) was quantitatively determined by FC/ET and the photometric dynamic coloration method toward ^99mTc-MDP was established. The photometric interference assay between bacterial endotoxins and Tachypleus Amebocyte lysate (Tal) on ^99mTc-MDP solutions of different concentrations was carried out, and the results showed that no interference was happened to spectrophotometry when the sample solution was diluted by 30 times, and the concentration of bacterial endotoxin was less than 0.600 EU/mL that was in accordance with the criteria of Chinese Pharmacopoeia (2020 version), which stipulates the concentration of endotoxin should be less than 15 EU/mL. Moreover, the bacterial endotoxin detection results of ^99mTc-MDP with different activities were not changed after decay. This established ^99mTc-MDP bacterial endotoxins quantitative detection method, which is easily operated, has good reproducibility and reliability that can be utilized for the quantitative assay of bacterial endotoxin of ^99mTc-MDP injection.