Phase I/II clinical trials using ricin A chain-containing immunotoxins (RTA-ITs) in <125 patients with lymphoma have established that vascular leak syndrome (VLS) is the dose-limiting toxicity. A similar side effect has also been described for other immunotoxins (ITs) and for cytokines, growth factors, antibodies, and chemotherapeutic agents. To better reproduce the conditions underlying vascular leak syndrome in patients treated with immunotoxins, human skin grafts were transplanted onto SCID mice and modifications in the graft were studied after systemic administration of a RTA-IT. Compared with mice receiving saline, an increase in wet/dry weight ratios of these grafts was observed in mice injected with RTA-IT. An increase in the permeability of the human blood vessels was also demonstrated by the extravasation of Carbon Black and the accumulation of Evans Blue dye in the graft. Taken together, these observations suggest that the RTA-IT can induce VLS-like manifestations. This model should facilitate the testing of potential inhibitors of VLS, which might reduce the toxicity of ITs and other therapeutic agents.