Hyperthermia is used as a new treatment modality for ocular melanoma. We wondered whether this treatment would affect the antigenicity of melanoma cells and studied the effect of hyperthermia on the expression of histocompatibility antigens (HLA), beta 2-microglobulin, as well as heat-shock proteins (HSP-60 and HSP-70) on choroidal melanoma cells. Uveal melanoma cell lines were exposed to different temperatures (39-45 degrees C) in a waterbath. Antigen expression was determined with fluorescence-activated cell sorting analysis, using monoclonal antibodies against HLA and HSP. In a 51Cr-release cytotoxicity assay we studied the effect of heat on natural killer (NK) cell susceptibility. Exposure to 45 degrees C for 30 min reduced expression of HLA class I antigens and beta 2-microglobulin. A greater reduction was observed after longer exposure times. Expression of HSP-70 was increased after exposure to 45 degrees C at all time intervals, while expression of HSP-60 was not induced by heat treatment. We did not find a significant difference in the NK cell susceptibility between heated and unheated cells. Hyperthermia has a time- and temperature-dependent effect on expression of HLA class I and HSP-70 molecules on the cell surface of uveal melanoma cells. Hyperthermia did not alter the susceptibility to NK cell lysis.