We give the weakest constraint qualification known
to us that assures the maximal monotonicity of the operator
A*o T o A when A is a linear continuous mapping between two reflexive Banach spaces and T is a maximal monotone operator. As a special case we get the weakest constraint qualification that assures the maximal monotonicity of the sum of two maximal monotone operators on a reflexive Banach space. Then we give a weak constraint qualification assuring the Brezis-Haraux-type approximation of the range of the subdifferential of the precomposition to A of a proper convex lower-semicontinuous function in non-reflexive Banach spaces, extending and correcting in a special case an older result due to Riahi.