Lyophilized E. coli endotoxin should be stored at 2-8°C. Endotoxin solutions are generally very stable upon storage except at more dilute concentrations. Rehydrated E. coli endotoxin should be stored refrigerated only (at 2-8°C and not frozen). This stock solution may be kept for up to four weeks. Further dilutions of E. coli endotoxin may begin to lose potency after a few days. The storage stability of dilute endotoxins may perhaps be related to the cleanliness of the glassware in which they are kept because endotoxin can “plate out” on irregular or dirty glass surfaces. Since some types of plastic have been shown to have an affinity for endotoxin, the use of plastic containers to prepare and store endotoxin must be approached with caution. Remember that the same precautions for technique, water and pipettes apply to endotoxin rehydration as apply to lysate rehydration.