Defining the factors and/or mechanisms that lead to the predominance of a particular CD4+ T-cell subset (Th-1 vs. Th-2) is an area of intense investigation. In murine schistosomiasis, Th-2-type T cells become predominant after deposition of eggs. The most immunoreactive egg components are glycoproteins. Previously we identified two interesting oligosaccharides found on schistosome eggs and schistosomula. One, lacto-N-fucopentaose III (LNFP-III) contains the interesting trisaccharide Lewisx, which is a weak ligand for P-selectin and is a sugar also found on the alpha and beta chains of the integrin lymphocyte function-associated molecule 1, a ligand for intercellular adhesion molecule 1. Because of the correlation between schistosome egg glycoproteins and Th-2 dominance, the present study examined whether LNFP-III and structurally related oligosaccharides were lymphostimulatory and/or able to induce factors known to down-regulate Th-1 cells. We found that LNFP-III and related sugars did induce proliferation of splenic non-T cells, B220+,CD4-,CD8- cells (B cells) of schistosome-infected and naive mice. In contrast to proliferation, LNFP-III was the only oligosaccharide that induced spleen cells to produce large amounts of interleukin 10 and prostaglandin E2, two molecules known to down-regulate Th-1 cells. Further, only spleen cells from infected mice produced cytokines after oligosaccharide stimulation. Interestingly, LNFP-III stimulation did not induce production of interleukin 4. Thus, a specific carbohydrate ligand has been identified that stimulates B cells to proliferate and produce factors that down-regulate Th-1 T cells. Further, we suggest that identical or structurally related ligands may contribute to the known Th-1 down-regulation in other parasitic diseases and in chronic blood-vascular diseases such as human immunodeficiency virus infection and a number of metastatic carcinomas and that this effect may, therefore, be a general phenomenon.