Twenty-two invasive adenocarcinomas of the large intestine were investigated histologically and immunohistologically to determine the localisation and composition of the tumor-infiltrating lymphoreticular cells: The lymphoreticular cells were consistently more frequent in the intervening stroma than in the tumor foci. Monocytes/macrophages (Mono 1+), helper/inducer cells (Leu-3 a+), and T lymphocytes (Leu-1+) were mostly observed in large numbers, thus constituting the great majority of tumor-infiltrating cells. In most tumors, the suppressor/cytotoxic T lymphocytes (Leu-2a+), plasma cells, eosinophils, and tissue mast cells occurred in moderate numbers. T-accessory cells bearing the CD 1 surface-antigen (Leu-6+) were generally detected in low, or even very low numbers, while B lymphocytes (TO 15+) and natural killer cells (Leu-7+) were nearly, and dendritic reticulum cells (KiM 4+) were totally absent from the lymphoreticular infiltrates. Though an immunohistological in situ analysis of tumor-infiltrating cells allows only limited conclusions concerning their functional properties, the often dense and polymorphic infiltrates might reflect a relatively intensive interaction between the specific and non-specific host defense and the malignant tumor.