Abstract: Objective To compare the endotoxin levels in plasmid purification with different plasmid DNA extraction kits. Methods Six commerical kits（QIAGEN Plasmid Mini Kit（12123），QIAGEN EndoFree Plasmid Maxi Kit（12362），CWBIO PurePlasmid Midi Kit（CW0502），EndoFree Plasmid Midi Kit，Endo-free Plasmid Mini Kit，Endo-free Plasmid DNA Mini Extraction Kit）were applied to purify plasmid，endotoxin levels were determined with the gel-clot technique and Limulus amebocyte lysate chromogenic method，respectively. Results The yields，A260 /A280 and A260 /A230 values of plasmid DNA extracted with the six kits were appropriate，the endotoxin levels were different，and the endotoxin levels extracted with endotoxin- free kits were relatively much lower. With QIAGEN EndoFree Plasmid Maxi Kit（12362），the endotoxin level was less than 0. 125 EU/μg. Conclusion The endotoxin levels in plasmid DNA extraction with six kits were significantly different，which shall be selected according to the actual situation.

Key words: plasmid, endotoxin, Limulus amebocyte lysate, plasmid extraction kit