Eosinophil granule proteins are deposited in cutaneous lesions in many human diseases, but how these proteins contribute to pathophysiology is obscure. We injected eosinophil cationic protein (ECP or RNase 3), eosinophil-derived neurotoxin (EDN or RNase 2), eosinophil peroxidase (EPO), and major basic protein-1 (MBP1) intradermally into guinea pig and rabbit skin. ECP and EDN each induced distinct skin lesions at >or=2.5 microM that began at 2 days, peaking at approximately 7 days and persisting up to 6 wk. These lesions were ulcerated (ECP) or crusted (EDN) with marked cellular infiltration. EPO and MBP1 (10 microM) each produced perceptible induration and erythema with moderate cellular infiltration resolving within 2 wk. ECP and EDN localized to dermal cells within 2 days, whereas EPO and MBP1 remained extracellular. Overall, cellular localization and RNase activity of ECP and EDN were critical for lesion formation; differential glycosylation, net cationic charge, or RNase activity alone did not account for lesion formation. Ulcerated lesions from patients with the hypereosinophilic syndrome showed ECP and EDN deposition comparable to that in guinea pig skin. In conclusion, ECP and EDN disrupt skin integrity and cause inflammation. Their presence in ulcerative skin lesions may explain certain findings in human eosinophil-associated diseases.