Sulfur mustard (HD) is a vesicant compound that was first used as a chemical warfare agent in World War I. (Papirmeister et al. 1991). Numerous animal models have been used to study HD-induced vesication. In this article, we describe modifications of the vapor cup model of Mershon and colleagues (1990) to establish a new vapor cup model for use in neonatal mice. The need to develop this model resulted from the development of gene-targeted knockout mice that can be used to evaluate the function of specific genes and their contribution to HD-induced pathology. However, the knockouts are haired mice; therefore, it is necessary to perform vapor exposures on the pups prior to their growing hair. Neonatal mice were anesthetized with isofluorane inhalation and placed in sternal recumbency on a 37 degrees C isothermal pad to maintain body heat during exposure. The vapor cup consisted of a 1.5-mL microfuge tube cap (8 mm inside diameter) modified using a Dremel tool to contour its rim to better fit the curve of a mouse pups back. The inside of the cap was fitted with an 8-mm disk of Whatman #2 filter paper, and the rim of the cap was coated with a thin bead of Thomas Lubriseal grease. Ten muL of neat HD was placed on the filter paper disk, and the cup was immediately inverted and placed onto the back of an anesthetized mouse pup. Exposure times varied from 10 to 30 min. At 24 h postexposure, the mice were euthanized; the HD-exposed skin was removed and fixed in 10% neutral buffered formalin. Following a minimum of 24 h of formalin fixation, the skin sections were bisected across the exposed area. The sections were embedded in paraffin with the central straight-cut surfaces being the focus of histological evaluation. The amount of damage associated with the HD vapor cup exposure varied with time in a dose response fashion. Typical damage consisted of varying amounts of epidermal necrosis at the basal cell level, with occasional separation of epidermis from dermis (microvesication). In severe cases there was complete coagulation of the epidermis and no microvesication. This model should prove useful in identifying the biochemical mechanism of action of HD and ultimately aid in the evaluation of treatment compounds. It may also provide a relevant exposure model for other compounds for which the assessment of vapor-induced damage is necessitated.