Glucans having exclusively or mainly β-1,3 linkages are used as immunogens. These comprise β-1,3-linked glucose residues. Optionally, they may include β-1,6-linked glucose residues, provided that the ratio of β-1,3-linked residues to β-1,6-linked residues is at least 8:1 and/or there are one or more sequences of at least five adjacent non-terminal residues linked to other residues only by β-1,3 linkages. The glucans will usually be used in conjugated form. A preferred glucan source is curdlan, which may be hydrolyzed to a suitable form prior to conjugation.