Commercial pharmaceutical waters were evaluated regarding their interference in the chromogenic Limulus Amebocyte Lysate (LAL) test. Two of the four tested waters strongly inhibited the U.S. Reference Standard Endotoxin, while a Control Standard Endotoxin was less affected. The inhibitory effect could be mimiced by adding trace amounts of certain metal ions to a non-interfering water. In conclusion, the water used in the LAL test must be chosen with great care.