The expression of intercellular adhesion molecule-1 (ICAM-1) in tumoral tissues may promote their interaction with the immune system and cytotoxic effect on tumoral cells. This observation led to the investigation of ICAM-1 expression and modulation in different tumoral cell systems in vitro. Recently, retinoic acid-responsive elements have been found in the 5'-regulatory region of the human ICAM-1 gene. In the present study, we investigated, by flow cytometry, the effect of retinoic acid on the surface expression of ICAM-1 in human thyroid carcinoma cell lines. Two papillary (NPA and TPC-1), one follicular (WRO), one anaplastic (ARO) and one immortalized fetal (TAD-2) cell line have been studied. All of them produced constitutively ICAM-1; its surface expression and specific messenger ribonucleic acid (mRNA) levels were increased significantly by retinoic acid in all except the WRO cell line. ICAM-1 hyperexpression by retinoic acid was time dependent, reversible, and dependent on mRNA and protein synthesis. Furthermore, cytokines, such as interferon-gamma and tumor necrosis factor-alpha, both individually and, to a greater extent, in combination with retinoic acid, increased ICAM-1 surface expression and its mRNA levels. In conclusion, retinoic acid is able to induce ICAM-1 up-regulation via mRNA accumulation in human thyroid carcinoma cell lines.