In search of factors governing the accumulation of tumor infiltrating lymphocytes (TIL), frozen sections from fresh surgical specimens of laryngeal carcinoma (n = 36) were tested by alkaline phosphatase-anti-alkaline phosphatase (APAAP) immunohistochemistry for monomorphic determinants of HLA class I and class II expression on tumor cells and for the distribution of lymphoid cells bearing CD differentiation antigens. Cell subsets were quantitated in two tumor compartments, tumor mass and tumor stroma, by computer-assisted image analysis. In a portion of examined samples lymphoid cell suspension was isolated from cancerous tissues and assessed by flow cytometry. It has been found that T cells, localized mostly in tumor stroma, were predominant cell population in the tumor microenvironment. Their ability to penetrate tumor mass but not tumor stroma, by CD8+ T cells in particular, but also by natural killer (NK) cells, was associated with HLA class I antigen expression on tumor cells. In flow cytometric analysis activated T lymphocytes (CD3+DR+) were abundant in HLA+ tumors as compared to HLA- ones. In 4 year follow up of 20 patients the mortality was higher in HLA- group but the data were not statistically significant. These results show that HLA class I expression on tumor cells favor penetration of cytotoxic lymphoid cells into tumor mass, at least in the laryngeal cancer.