A washing agent dispenser for machines for washing includes a dispenser body with a receptacle for containing a washing agent, a lid having a lid body mounted in a guided way with respect to the dispenser body for being movable between a position of closing and a position of opening of the receptacle, and sliding guides between the dispenser body and the lid body. The sliding guides include: a first guiding element and a first guided element; and a second guiding element and a second guided element. At least one of the dispenser body and the lid body includes at least one constraint element, which is additional to the first walls and second walls and is pre-arranged for limiting displacements of the guided elements with respect to the guiding elements in a direction generally transverse to a direction of sliding of the lid determined by the sliding guides.