Inflammatory cells (the term is considered to include lymphocytes, plasma cells, macrophages, mast cells and PMN) were identified and quantitated in sections from human carcinomas. Two types of cellular infiltrates are described. In the stroma surrounding the cancer tissue (peritumoural) and, to a lesser degree, also within the malignant tissue (intratumoural), lymphocytes, plasma cells, macrophages and mast cells dominated, whilst numerous polymorph nucleated cells (PMN) and aggregates of macrophages characterized central tumour necroses. The demonstration of both T-lymphocytes and macrophages in and around the cancer tissue supports the view that a local immune reaction is initiated by the tumour, and the consistent finding of IgG, IgA and IgM plasma cells at the borders of the carcinomas, indicates that plasma cells are also part of an immune response at the tumour site. This view is further supported by the demonstration of an increased proportion of IgG containing plasma cells in the stroma of colon tumours, compared with normal mucosa.