We have investigated the in vitro expression of membrane and soluble intercellular adhesion molecule-1 (ICAM-1) by human thyroid cells from 20 patients with Graves' disease and 5 normal subjects. Membrane ICAM-1 was not detected by flow cytometry analysis in non-cultured thyrocytes from either normal or Graves' disease tissues. It appeared on thyroid cells after a 24-h culture in monolayers and showed a regular dose-dependent increase. The same results were obtained with soluble ICAM-1 (sICAM-1) in culture media from cells cultured in monolayers, vesicles or follicles. No change was obtained with different concentrations of fetal calf serum added to the media. Coculture of Graves' disease thyrocytes with autologous peripheral blood lymphocytes (PBL) or intrathyroidal lymphocytes (ITL) enhanced the expression of both membrane and sICAM-1 whatever the culture model. When normal thyrocytes were cocultured with PBL, sICAM-1 increased but with ITL sICAM-1 remained unchanged. High concentrations of gamma interferon induced an increase of both membrane and sICAM-1 in the three culture models. However the increases were greater with vesicles and follicles. Only sICAM-1 levels were raised with 0.1, 1 and 10 microM retinoic acid. These results suggest that ICAM-1 appears in culture, possibly due to mechanical effects such as adherence to plates and cell-to-cell contacts. Moreover, its expression is modulated by several factors such as cytokines or retinoic acid. Further investigations are needed to establish whether ICAM-1 is really involved in the pathogenesis of Graves' disease.