The results of Phase I/II clinical trials indicate that ricin A-chain-containing immunotoxins cause vascular leak syndrome, characterized by hypoalbuminemia with resultant weight gain and edema. Vascular leak syndrome may be a dose-limiting factor during treatment with ricin A-chain-containing immunotoxins. In this report, we determined the effect of ricin A-chain and ricin A-chain-containing immunotoxins on human umbilical vein endothelial cells with the aim of developing an in vitro model to study vascular leak syndrome. The major findings of our study are: (1) Human umbilical vein endothelial cells undergo rapid and dramatic changes in morphology after treatment with ricin A-chain and ricin A-chain-containing immunotoxins. These changes include rounding of the cells and, eventually, the formation of gaps between them. (2) The permeability of human umbilical vein endothelial cell monolayers to passage of molecules increases after exposure to ricin A-chain or ricin A-chain-containing immunotoxins and this is consistent with the morphologic changes. (3) Human umbilical vein endothelial cells bind 125I-rRTA in a dose-dependent manner but binding is not specific. (4) Human umbilical vein endothelial cells are moderately more sensitive to ricin A-chain-induced inhibition of protein synthesis and proliferation than simian virus-transformed mouse endothelial cells. (5) The morphologic changes are observed 1 h after exposure to the toxins, whereas inhibition of protein synthesis is not detectable until 4 h after a similar exposure. The in vitro model represents a first step in dissecting the complex events which occur in cancer patients who develop vascular leak syndrome after treatment with ricin A-chain-containing immunotoxins.