Herein is reported a method for determining bacterial endotoxin at low concentrations in a sample of an antibody (that has been produced using bacterial cells) comprising the following steps in the following order: i) adding magnesium ions to the sample, ii) diluting the sample, iii) dialyzing the sample having a pH-value of 5.7-8.0 against an endotoxin-free aqueous solution, and iv) determining bacterial endotoxin in the sample using a bacterial endotoxin test, particularly the limulus amoebocyte lysate assay.