Six thymomas were studied with monoclonal and polyclonal antisera and an immunohistochemical technique to characterize the lymphocytic and epithelial components of the tumor. The T lymphocytes had characteristics of thymocytes, and cortico medullary differentiation was demonstrated within all tumors, regardless of histologic appearance. Cortical thymocytes were positive with anti-terminal deoxynucleotidyl transferase, OKT6, and OKT10, all of which failed to stain medullary thymocytes. Conversely, A1G3 stained medullary thymocytes exclusively. Epithelial cells reacted with A2B5, an antibody that identifies cells of neuroendocrine origin. Epithelial cell processes were also positive with anti-p19, a monoclonal antibody directed against the internal protein of HTLV, which also reacts with normal thymic epithelium. No correlation was found in the distribution of epithelial cells with evidence of cortical and medullary thymocyte differentiation. However, the components of the tumor mirrored the normal cellular and structural components of the thymus.