The reaction of [Mn-12(CH3COO)(16)(H2O)(4)O-12] center dot 2CH(3)COOH center dot 4H(2)O, Na-8[HPW9O34] center dot 24H(2)O, K2HPO4, and ethylenediamine hydrochloride in an aqueous solution leads to the isolation of a new polyoxometalate-based mixed-valent manganese aggregate, K14Na17[((Mn13MnO12)-Mn-III-O-II(PO4)(4)(PW9O34)(4)] center dot similar to 56H(2)O (1). Single-crystal X-ray diffraction indicates that the polyoxoanion of 1 exhibits a mixed-valent [(Mn13MnII)-Mn-III(mu(2)-O)(6)(mu(3)-O)(6)(mu(4)-PO4)(2)(mu(5)-PO4)(2)(m u(5)-PO4)(2)](5+) complex encapsulated by four [B-alpha-PW9O34](9-) trivacant Keggin-type moieties [crystal data for 1: monoclinic, C2/c (No. 15), a = 36.341(7) angstrom, b = 18.325(4) angstrom, c = 36.668(7) angstrom, beta = 119.24(3)degrees, V = 21308(7) angstrom(3), Z = 4]. In a magnetic point of view, all of these paramagnetic manganese aggregates in 1 are well wrapped into the diamagnetic inorganic shells. Variable-temperature, solid-state, direct-current susceptibility measurements show that compound 1 exhibits strong antiferromagnetic interactions inside the [Mn-14] core.