  We consider a group SO(2n+1) over a p-adic field, and tempered irreducible
representations of this group, of unipotent reduction. We use the construction
due to Lusztig of these representations. In an old paper with Moeglin, we have
defined an involution in the complex vector space generated by those
representations which are elliptic. It is strongly related to another
involution defined by Lusztig for finite groups. We give a new definition of
our involution and we prove it commutes, in some sense, with Jacquet functor.
