Immunization of mice with Saccharomyces boulardii crude extract caused a significant reduction in Schistosoma mansoni worm burden (23.7%; P = 0.02) in comparison to control mice. Mice sera collected after the 1st immunization showed a significant increase (P < 0.05) in both IgM and IgG against parasite soluble cercarial antigenic preparation (CAP) than unimmunized mice sera (UMS). Upon 2nd immunization, sera from yeast extract immunized mice (YIM) showed increase of IgM and IgG (1.6 and 1 fold) against CAP than UMS. Sera collected 2, 4 and 6 weeks (W) after infection of YIM with S. mansoni cercariae showed higher IgM (1.4, 1.5 and 1 folds) and IgG (0.9, 1.2 and 1.1 folds) levels against CAP than sera of infected UM (IUM). When compared to sera from UM, a week post 1st and 2nd immunization sera showed a significant elevation (P < 0.05) of both IgM and IgG to parasite soluble adult worm antigenic preparation (SWAP). Sera collected at 2, 4 and 6W after infection of YIM with parasite cercariae showed higher IgM (1.1, 1.3 and 1.2 folds) and IgG (1.2, 1.1 and 1.3 folds) reactivities against SWAP than sera of IUM. One week post 1st and 2nd immunization with the yeast extract, sera showed a significant increase (P < 0.05) of both IgM and IgG levels against parasite soluble egg antigen (SEA) when compared with sera from UM. At 2 and 4W post infection, sera from immunized infected mice showed a significant decrease in IgM (P < 0.05) but the decrease of IgG (0.7 fold) was not significant when compared to IUM. Six weeks post infection the decrease in both antibodies remained but it was significant for IgG (P < 0.05) but not for IgM (2.1 folds) when compared with IUM. Immunization with S. boulardii extract caused a non-significant increase (1.2 folds) in mesenteric lymph node (MLN)-CD4+T cells, significant elevation of MLN-CD8+T cells (P < 0.05) and a non-significant increase (0.9 folds) of B-cells when compared to that of UM. Also, Immunization with S. boulardii extract caused 1 fold elevation in both CD4+ and CD8(+)-T lymphocytes in thymus of YIM when compared with their counts in thymus of UM.