Apparently complex modulatory effects of alpha-interferon (alpha-IFN), tumor necrosis factor (TNF), and epidermal growth factor (EGF) have been found in neoplastic human thyroid cells, which could possibly affect the final outcome in neoplastic disease. This was achieved by examining the influence of alpha-IFN, TNF, and EGF alone and in combination, on human leukocyte antigen-DR (DR) antigen expression and viability of neoplastic and non-neoplastic human thyroid cells in culture. alpha-IFN-induced DR antigen expression on non-neoplastic human thyroid cells, whereas TNF-alpha or EGF alone were ineffective. The addition of the same TNF-alpha concentrations (10 to 100 ng/ml) to alpha-IFN enhanced the expression of DR antigens compared with the effect of alpha-IFN alone. However, EGF inhibited alpha-IFN-induced DR on the same cells and at the same concentrations (10 to 500 ng/ml) at which the growth factor alone was ineffective. In contrast to the common pattern of cytokine effects on DR expression of all nonmalignant thyroid cell lines, neoplastic thyroid cell lines showed divergent responses to alpha-IFN, TNF-alpha, and EGF. In three malignant thyroid cell lines that were DR negative (follicular carcinoma WRO 82-1 and NRO 87-1 cell lines, and anaplastic carcinoma ARO 81-1), DR antigen could be induced by alpha-IFN and enhanced by TNF-alpha, whereas EGF was ineffective. In a fourth cell line (an anaplastic carcinoma SW1736) alpha-IFN, TNF-alpha, and EGF alone were capable of inducing DR, and a combination of either TNF-alpha and EGF with alpha-IFN potentiated DR induction. In a fifth neoplastic cell line (papillary carcinoma, NPA) that constitutively expressed surface DR, its expression was inhibited by both alpha-IFN and TNF-alpha and was not affected by EGF.