The majority of patients dying from malignant disease are killed by metastases rather than by the primary tumour mass. The possible influence of immunological factors upon the progress of established tumours in both patients and experimental animals is receiving increasing attention and has been much discussed (Morton, 1973). However, less attention has been paid to the role, if any, that these factors play in the initial arrest of metastatic cells or their pattern of localization in different organs. In the present studies, isotopically labelled tumour cells were introduced into the systemic circulation of mice as a model of the haematogenous dissemination of metastatic cells, in situations where the host is sensitized to tumour antigens. In addition, since neuraminidase treatment has been shown to influence the organ distribution of transformed normal cells (Woodruff and Gesner, 1969), experiments were also included to examine the effects of neuraminidase on the pattern of localization of injected tumour cells in animals with different tumour-bearing status. MATERIAL AND METHODS TumoursA fibrosarcoma (MC-1) originally induced by methylcholanthrene was routinely passaged as an ascites tumour in syngeneic C3H/He Ha female