Previously, a two-part interlaboratory round robin endotoxin assay study was completed. This first study showed that intralaboratory results had small variation but that intra-laboratory results had very high variation. This held true for both parts of the study; but in the second part of the study, when the extraction protocol was standardized, the inter-laboratory results showed a lower variation, which suggested that with further standardization, further reduction of differences between laboratories might be achieved so that results between laboratories would become comparable. The results stimulated interest in extending the study to include cotton dust with two levels of endotoxin, standardization of the extraction protocol and assay with the assay kits all from the same production lot. The results of a second round robin endotoxin assay study where a common assay protocol using the same endotoxin assay kit is reported.