The test substance, hexamethylene diisocyanate, was tested in the mouse micronucleus assay in collaboration with MPI Research L.L.C. Information on the experimental design and methodology is provided in the attached report provided by MPI Research L.L.C (Appendix III). All in vivo portions of the study, including inhalation exposures were performed by MPI. Animal euthanization, bone marrow collection, slide preparation and evaluation were performed by MA BioServices, Inc. The assay was performed in two phases. The first phase, designed to set exposure levels for the definitive study, consisted ofa preliminary toxicity study (range finding study). The second phase, the micronucleus study, evaluated the potential of the test substance to increase the incidence of micronucleated polychromatic erythrocytes in bone marrow of male and female mice. In both phases of the study, test and control articles were administered by a single 6 hour whole body inhalation.