Extracts f rom the salivary glands of the bloodsucking leech Hirudo medicinalis are used in topical medicat ions to treat venous disorders. Contac t allergy to crude leech extract, which contains hirudin and other proteins such as eglins, bdellins, and hyaluronidase has been reported, l, 2 Hirudin is a potent selective thrombin inhibitor and may therefore be of value in the t rea tment of thrombot ic diseases? Recombinan t desulfato-hirudin (rH), p roduced in Saccharomyces cerevisiae cultures, has a molecular weight of 6964 d and is practically identical to the natural hirudin variant 1, lacking only the sulfate moiety on tyrosine 63. 4 Several pharmacologic and clinical trials with rH have been performed, so far with few adverse experiences such as hypersensitivity reactions 4 or hemorrhages. 5 We repor t a volunteer participating in Clinical Pharmacology studies with rH who exper ienced acute, probably IgG-media ted , urticaria on reexposure to hirudin.