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BCG (Glaxo) and C. parvum (Wellcome CN 6134) have been examined for suppression of growth of a range of syngeneically transplanted rat tumours, both carcinogen-induced and of spontaneous origin. Treatment by locally applied adjuvant controlled growth of both immunogenic and non-immunogenic tumours, and the response to highly immunogenic tumours was not abolished by host immunosuppression with whole body irradiation known to abrogate induction of tumour-specific immunity. In addition, rat tumour xenografts in congenitally athymic mice were suppressed by admixture with BCG or C. parvum. In contrast to these findings in immunoincompetent animals, host phagocytic cell depletion, by systemic administration of silica, a known macrophage poison, abrogated adjuvant contact suppression of tumours in syngeneic rats and athymic mice. These findings suggest that tumour suppression by regionally applied adjuvants may be more dependent upon local activation of host macrophages than upon generalized stimulation of lymphocyte mediated responses. This is further supported by the market correlation, within a range of tumours, between susceptibility to regionally applied BCG and normal levels of host macrophage infiltration, and in addition the facilitation of tumour suppression achieved by macrophage enrichment of tumour cell: BCG inocula.