Abstract: In this paper,three kinds of tissue deposits of immunr complexes(TIC) and the S100 protein positive histiocytes were investigated in the lesions of 21 patients with erythema induratum(EI) using DIF and ABC immuno-histochemical method.DIF showed that TIC were deposited on or around the blood vessel walls in 19 patients and mainly were TIC IgM.ABC immunohistochemistry showed that the S100 positive histiocytcs were present in the lymphohistocytic infiltrates of the deep dermis and subcutaneous layer in all patients and its cmunhcr was related to the duration of lesions.These results suggest that EI is an immune complex type vasculitis,and type Ⅲ and Ⅳ hypersensitivity play important roles in the pathogencsis of EI.