The relative content of lymphocytes and macrophages was studied in 20 solid malignant human tumours. Mechanically prepared tumour-cell suspensions contained both lymphocytes (0.2–4.0%) and macrophages (0.2–7.0%). Macrophages were characterized as esterase-positive, phagocytosing cells. The distribution of the cells within the tumours was studied on cryostat sections. A modified esterase method, using α-naphthyl butyrate as substrate, always gave an intensive staining of tissue-infiltrating macrophages, even in malignant tissues where some neoplastic cells exhibited a weak to moderate esterase activity. Tumour-infiltrating lymphocytes were studied in HE-stained tissue sections. The study demonstrates that both cell types were usually more numerous in the stroma surrounding the tumour tissue and in the stromal septa between the cords of malignant cells (peripheral infiltration) than in the central areas of the tumours. No signs of cell necrosis were seen near the lymphocyte zones or the stromal macrophages; however, some central necrotic areas contained aggregates of macrophages, lymphocytes and PMN.