A Banach space X is said to have property (µ s ) if every weak *null sequence in X * admits a subsequence such that all of its subsequences are Cesàro convergent to 0 with respect to the Mackey topology. This is stronger than the so-called property (K) of Kwapień. We prove that property (µ s ) holds for every subspace of a Banach space which is strongly generated by an operator with Banach-Saks adjoint (e.g. a strongly super weakly compactly generated space). The stability of property (µ s ) under ℓ p -sums is discussed. For a family A of relatively weakly compact subsets of X, we consider the weaker property (µ s A ) which only requires uniform convergence on the elements of A, and we give some applications to Banach lattices and Lebesgue-Bochner spaces. We show that every Banach lattice with order continuous norm and weak unit has property (µ s A ) for the family of all L-weakly compact sets. This sharpens a result of de Pagter, Dodds and Sukochev. On the other hand, we prove that L 1 (ν, X) (for a finite measure ν) has property (µ s A ) for the family of all δS-sets whenever X is a subspace of a strongly super weakly compactly generated space.2010 Mathematics Subject Classification. Primary: 46B50. Secondary: 47B07.