A two part interlaboratory endotoxin assay study is underway. 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 results from the first phase of the study have been collected and the first hand interpretation will be reported here. The intralaboratory variations were small; but large and significant interlaboratory variation was observed.