Abstract: Immunocompetent cells infiltrated into the thyroid carcinoma (30 cases) and benign thyroid lesions (21 eases) were studied by monoelonal, antibodies (Leu4, OKT4, OKT8, B1, M718) with ABC method. The majority of mononuclear cells (MNC) infiltrated into the thyroid carcinomas were T cells and macrophages and the minority was B cells. The ratio of OKT4/OKT8 was 1.35. The existence of M718~+ multinucleated macrophages in the tissues was one of the characters of thyroid carcinoma. In contrast, MNC in benign lesions was rare. We suggested that the infiltrating of MNC and multinucleated macrophages were related with the types of carcinoma and the prognosis of patients.