Recently the combined therapy with interferon-alpha (IFN-alpha) and cimetidine has been reported to be effective against advanced renal cell carcinoma (RCC). IFN-alpha and cimetidine have an antitumor effect partly due to enhancement of cytotoxic activity of lymphocytes against cancer cells. We examined the expression of major histocompatibility complex (MHC) antigens and adhesion molecules on 4 fresh RCC cells and 5 RCC cultured cell lines, which have an important role in recognition and killing of cytotoxic lymphocytes against cancer cells. The effect of treatment with IFN-alpha and/or cimetidine on the expression of MHC antigens and adhesion molecules on RCC cells was also investigated. MHC class I and leukocyte function-associated antigen-3 (LFA-3) were expressed on all RCC cells, but not MHC class II. Intercellular adhesion molecule-1 (ICAM-1) and B7 were expressed on 6 and 5 of 8 RCC cells, respectively. IFN-alpha significantly augmented the expression of MHC class I in 6 of 9 RCC cells, ICAM-1 in 1 and LFA-3 in 2 of 8 RCC cells. However, IFN-alpha did not affect the expression of MHC class II and B7. On the other hand, cimetidine enhanced the expression of LFA-3 in 2 of 8 RCC cells, but not MHC antigens, ICAM-1 or B7. The combination of IFN-alpha and cimetidine did not show a synergistic enhancing effect on the expression of MHC antigens, ICAM-1, LFA-3 or B7. These results suggest that IFN-alpha augments the sensitivity of RCC cells to lysis by cytotoxic lymphocytes partly due to the enhancement of expression of MHC class I, ICAM-1 and LFA-3 on RCC cells, and that cimetidine also augments the susceptibility of RCC cells to lymphocytes by the enhanced expression of LFA-3 on RCC cells.