The two part interlaboratory round robin endotoxin assay study has been completed. In both parts of the study, filter membranes with the same approximate amount and type of cotton dust were sent for analysis to laboratories that ‘routinely' perform endotoxin analyses. Each of these laboratories performed the analysis using the methodology common to their laboratory. In the second part of the study, filter membranes with cotton dust were again sent to the same laboratories where the analyses were performed as before but with a common extraction protocol. The highlights from the two-part study will be summarized. The results stimulated interest in extending the study to include cotton dust with three levels of endotoxin on two different types of filter membranes.