The product of the Lymphocyte Activation Gene-3 (LAG-3, CD223) is a high affinity MHC class II ligand expressed by activated CD4(+) and CD8(+) T cells, which can associate with the T cell receptor (TCR) and downregulate TCR signalling in vitro. We have also reported that a soluble mLAG-3Ig fusion protein works as a vaccine adjuvant in vivo in mice, enhancing Th1 and CD8 T cell responses. Here, we report that LAG-3 expression was found, using fluorescent activated cell sorting (FACS) analysis, on 11-48% of human tumour-infiltrating lymphocytes (TILs) isolated from eight freshly dissociated renal cell carcinomas (RCCs), and was restricted mostly to CD8(+) cells. Immunohistochemical analysis confirmed LAG-3 expression by TILs in 9/11 RCCs, as well as in tumours of different origins, such as melanomas (3/5) and lymphomas (7/7). Since not only antigen presenting cells (APCs), but also TILs themselves strongly express major histocompatibility complex (MHC) class II, we firstly investigated whether LAG-3/MHC class II T-T cell contacts might influence tumour cell recognition. However, cytotoxicity inhibition was not observed in two RCC-specific CD8(+) T cell clones in the presence of the LAG-3-specific MAb, and there was also no observed difference in the recognition of LAG-3-transfected or wild-type RCC by these cytotoxic T lymphocytes (CTLs). In contrast, MHC class II engagement by LAG-3Ig was found to enhance the capacity of immature dendritic cells to stimulate naive T cell proliferation and IL-12-dependent IFN-gamma production by T cells in vitro. These results therefore provide support for a role for TIL-expressed LAG-3 in the engagement of class II molecules on APCs, thereby contributing to APC activation and Th1/Tc1 commitment, without downregulating cytotoxicity.