Enhancement of infection and lowering of the complement level have been demonstrated in mice injected with a heat-treated suspension of baker's yeast (). The leucocyte response to the infection was not affected. The yeast preparation showed, , an intense anti-complementary activity on mouse serum and interfered with the microbial killing function of the mouse peritoneal macrophages. No significant stimulation of the growth of in the presence of the yeast was observed. The enhancement of the infection in mice treated with the yeast seems to be mediated, mainly, by complement depletion.