Investigation of oral administration of Saccharomyces boulardii in healthy volunteers demonstrates several cellular and humoral changes in peripheral blood. Among its effects are the increase of erythrocytes, leucocytes, polymorphs, neutrophils, complement components C3, C5, C3d, serum anticomplementary activity and leucocyte chemokinesis, specially when autologous serum and antigen have been added to the culture medium and decrease of complement haemolytic activity (CH50, classic and alternative pathways). We have also demonstrated that in vitro S. boulardii was able to activate complement directly, to fix C3b to its surface and that its phagocytosis by mononuclear cells was complement-dependent. The overall changes in serum proteins suggested changes of acute phase proteins typical of an inflammatory process. Furthermore S. boulardii had no mitogenic response of lymphocyte populations. Our results demonstrated that S. boulardii activates the reticuloendothelial system and complement system and suggest that S. boulardii merits therapeutic trial in a variety of clinical situations.