In patients undergoing immunotherapy for metastatic melanoma, the clinical response in immunotherapeutic trials may be partial or difficult to detect. Tumor metastasis biopsy allows direct characterisation of an anti-tumor immunological response. During a phase I/II trial of granulocyte macrophage colony stimulating factor (GM-CSF) transduced autologous melanoma immunotherapy, the cellular response was examined by immunohistochemical analysis in a limited number of tumor biopsies taken from patients who either responded or progressed. Clinical response was associated with tumor infiltration by CD4+ and CD8+ T-cells, macrophages and differentiated dendritic cells (DC), and expression of HLA-DR by the tumor cells. This tumor infiltration was associated with increased melanoma-specific peripheral blood precursor cytotoxic T-lymphocyte (pCTL) and the ability to obtain tumor-infiltrating lymphocytes in vitro. In contrast, progression or a lack of clinical response was associated with a lack of T-cell and DC infiltration into the tumor tissue in all such biopsies. Macrophages and eosinophils infiltrated these tumors, while T-cells and DC were present at some distance from the tumor. These preliminary data strongly suggest that the location and extent of T-cell and DC infiltration, as well as the expression of HLA-DR by tumor cells are associated with a clinical response in this form of melanoma immunotherapy.