Neoplastic tissue was obtained at operation from 10 renal cell carcinomas, from the adjacent 'normal' kidney in 6 cases and from 1 other normal kidney. The biopsies were snap frozen in liquid nitrogen and sections were subsequently stained with monoclonal antibodies against major histocompatibility complex (MHC) antigens, class I and II, and several types of mononuclear cell, by the indirect immunoperoxidase method. The degree of staining or the number of cells stained was estimated as heavy 4, through moderate 3, few 2, occasional 1, or nil 0. MHC Ag were consistently expressed, grade 2-4, by the glomeruli and proximal convoluted tubules of normal kidney, but were absent in 8 of 10 carcinomas. There was a grade 3-4 mononuclear cell infiltration in the stroma of normal kidney and between the carcinoma cells which was composed principally of macrophages. However in the two carcinomas expressing MHC Ag there was also a grade 2-3 infiltration with T lymphocytes. The absence of MHC Ag on carcinoma cells mitigates against attempts to potentiate the patient's immune response to his tumour, e.g. by renal artery embolisation.