Cryostat sections of 37 renal cell carcinomas (RCC)--25 clear cell type, 10 granular and 2 chromophobe--were studied with indirect immunoperoxidase method applying monoclonal antibodies (MoAb) to HLA-DR, -DP and -DQ antigens for analysis of HLA class II antigens, and anti-CD14, -CD3, -CD4 and -CD8 MoAb for tumor infiltrating mononuclear cells (TIM). Number of positive cells was estimated semiquantitatively and results of immunohistochemical investigation were correlated with clinical (patient age and sex, tumor size and TNM stage) and histopathological (cytology, histology, grade) characteristics of RCC. All RCC expressed HLA-DR, 92% -DQ and 73% -DP antigens with level of expression in hierarchy- DR>-DQ>-DP, but no statistically important correlation could be established with any of the histopathological or clinical parameters analyzed. Monocytes were more abundant than T lymphocytes and CD4+ than CD8+ T cells, whereas tumors with T lymphocyte predominance and approximately equal number of CD4+ and CD8+ T cells had greatest average diameter. Inadequate activation of T lymphocytes by tumor cells (despite capability of antigen presentation) could be the reason for association of parameters which indicates more aggressive tumor behavior with aberrant HLA class II antigen expression on RCC.