The major dose-limiting toxicity associated with ricin-conjugated immunotoxins (ITs) is vascular leak syndrome (VLS). IT-induced VLS appears to be dose dependent, occurs 2–4 d posttreatment (1 –6 ), and is thought to be a result of the toxin moiety, rather than the targeting moiety, because VLS is associated with ITs containing targeting moieties with different specificities. In addition, whole antibodies and FAb fragments can cause VLS, suggesting that this toxicity is not a result of Fc receptor binding or complement fixation (1 – 6 .