The authors review the statistical principles of parallel line bioassay and describe a computer program written to facilitate a correct approach to most coagulation assays. This program should be especially useful for those assays in which linearization of dose-response curves may prove difficult, or in which careful analysis of assay validity is required. How the knowledge of bioassay theory can be applied to von Willebrand factor Ristocetin Cofactor activity measurement is shown as a general example.