The interplay between the reticuloendothelial system and the growth of tumours is complex. Tumours stimulate the output of monocytes from the bone-marrow leading in some instances to a marked monocytosis. While tumours contain varying numbers of normal macrophages, and in some rat sarcomata they may constitute up to 60% of the total cells of the tumour, the monocytes in tumour bearers are abnormal in so far as they fail to enter sites of inflammation probably because they have bound immune complexes. There is an inverse correlation between the macrophage content of animal tumours and their capacity to metastasize and this may be related to the capacity of macrophages to kill tumour cells as a result of cell-to-cell contact. Macrophage cytotoxicity can be immunologically specific and this, co-operation with T-lymphocytes is required. In addition, macrophages can also be induced to express an immunologically non-specific anti-tumour cell activity.