Monoclonal antibody against recombinant human tumour necrosis factor alpha (rTNF) was used for the immunochemical detection of TNF in human blood monocytes and monocytic cell line U 937. Cells stimulated with phorbol myristate acetate (PMA) showed strong surface but not cytoplasmic staining. Unstimulated cells demonstrated weak or no staining. At early time after stimulation (1-2h) a spot reaction was seen in the Golgi area of the cytoplasm of stimulated cells. Coculture of tumour cells with monocytes also resulted in the induction of membrane TNF. Ultrastructural studies confirmed TNF localization within the cell membrane. These results indicate that TNF can be detected within the cells by immunocytochemistry which may make feasible studies on TNF appearance in cellular infiltrates in the tissues.