A collaborative study was undertaken to determine the potency in endotoxin Units (EU) and International Units (IU) of a control standard endotoxin, LIF-1. Five laboratories from the Swedish Pharmaceutical Industry participated in the study. As reference preparations, two official standards, USP reference standard endotoxin, EC-5 (expressed in EU) and WHOs international standard endotoxin (expressed in IU), were used. The study was performed using the Limulus Amebocyte Lysate (LAL), gel-clot test. The test protocol included dilutions of the endotoxin in steps of 1:1.25 instead of the conventional 1:2 step dilution method. This gave more precise and standardized results. The content of one vial of LIF-1 (CSE) was calculated to be 22 EU and 16 IU which indicates that 1 EU (USP) corresponds to 0.7 IU (WHO).