The pathogenic mechanisms underlying the development of autoimmune hepatitis (AIH) are still unclear. Since AIH is associated with the presence of various autoantibodies and certain HLA subtypes, it is likely that T and B cells play a major role in this disease. In this study we have determined the functional capacities of in vivo preactivated liver-infiltrating T cells (LTC) from patients with AIH. As controls we used LTC from patients with non-autoimmune hepatitis (non-AIH). Our results show that preactivated LTC from patients with AIH predominantly (190/255 clones) reside in the CD4+ population, whereas LTC in non-AIH are dominated by the CD8+ phenotype (148/254 clones). In view of this finding we have investigated the cytokine secretion patterns of 102 randomly chosen CD4+ T cell clones from six patients with AIH. As controls we have used 58 CD4+ LTC from 11 patients with non-AIH. All clones were stimulated by lectin and irradiated accessory cells and subsequent cytokine production was evaluated. LTC from patients with AIH have a lower interferon-gamma (IFN-gamma)/IL-4 ratio compared with LTC from non-AIH. Although clones from some patients with AIH produced very high amounts of IL-4 in vitro, this was not a constant finding. These results show that in vivo preactivated LTC from patients with AIH are mostly CD4+ T cells that produce more IL-4 than IFN-gamma. In contrast, LTC from patients with non-AIH are dominated by CD8+ and CD4+ T cells that produce significantly less IL-4 than IFN-gamma. Thus, liver-infiltrating T cells from patients with AIH and non-AIH belong to different functional T cell subsets. This may have implications for the regulation of humoral and cellular immune responses in inflammatory liver disease.