In mice killed Rothia dentocariosa cells in doses of about 1.5 mg dry weight activated anti-infection immunity to Listeria antigens and anti-tumour immunity to the ascitic form of mouse sarcoma S-180. Their probable target site is the macrophage. The Rothia-activated macrophages in human gingiva may take part in the pathogenesis of periodontal disease. Three models were employed to verify the immunostimulating properties of preventively administered Rothia dentocariosa bacterin-1) a spleen macrophage migration test, using mice immunized with Listeria innocua, with the soluble listeria Ei antigen as the antigenic signal, 2) determination of the increase in the Listeria monocytogenes LD50 for mice and 3) the prolongation of survival of mice carrying the S-180 tumour. In all three cases, the administration of Rothia bacterin stimulated the immune response to the later administration of other antigens. Furthermore, in the macrophage migration inhibition test, the chemotaxis of non-immune mouse macrophages was found to be stimulated. This gives evidence of the fact that Rothia bacterin has an activating effect on these macrophages.