All endotoxins from the gram negative microorganisms studied sofar contain lipid A, which is considered to be responsible for the biological activity. Endotoxins may therefore react in a similar way in quantitative assays. The varying non-lipid A part of different endotoxins may potentiate or reduce the activity of the identical lipid A part without changing the slope of the log-dose vs. response curve of these endotoxins. To test whether 13 endotoxins from different microorganisms react similarly with amoebocyte lysate (LAL) log-dose response curves were made using a chromogenic substrate method. A balanced incomplete block design with 13 blocks and four replications of each endotoxin was applied. A group of eight endotoxins with a common slope was defined with the Student-Newman-Keuls procedure. Two independent potency estimates of these endotoxins were in good agreement.