We have established a hepatocarcinoma cell line (LFCI2 A) that produces voluminous tumors when injected subcutaneously into syngeneic Commentry rats. These neoplastic cells express both insulin-like growth factors (IGF) I and II. When transfected with an episomal cassette-expressing IGF-I antisense RNA, the modified LFCI2 A cell lines become poorly tumorigenic and, when injected subcutaneously, are associated with inhibition of the growth of the parental tumoral cells and/or induction of regression of established tumors. By contrast, cell lines isolated after transfection with the IGF-II antisense-expressing vector were as tumorigenic as the parental cell lines. The results are discussed in terms of protective immunity induced by the tumoral cells transfected by the IGF-I antisense vector. In the transfected hepatocarcinoma cells that do not produce IGF-I, the expression of major histocompatibility complex class I antigen was increased at least 4-fold compared with parental cells. The introduction of these cells in vivo induced a tumor-specific immunity that was associated with CD8 T cells.