The largest single-molecule magnet (SMM) to date has been prepared and studied. Recrystallization of known [Mn12O12(O2CCH2But)(16)(H2O)(4)] (1; 8Mn(III), 4Mn(IV)) from CH2Cl2/MeNO2 causes its conversion to [Mn30O24(OH)(8)(O2CCH2But)(32)(H2O)(2)(MeNO2)(4)] (2; 3Mn(II), 26Mn(III), Mn-IV). The structure of 2 consists of a central, near-linear [Mn4O6] backbone, to either side of which are attached two [Mn13O9(OH)(4)] units. Peripheral ligation around the resulting [Mn30O24(OH)(8)] core is by 32 (BuCH2CO2-)-C-t, 2 H2O, and 4 MeNO2 groups. The molecule has crystallographically imposed C-2 symmetry. Variable-temperature and -field magnetization (M) data were collected in the 1.8-4.0 K and 0.1-0.4 T ranges and fit by matrix diagonalization assuming only the ground state is occupied at these temperatures. The fit parameters were S = 5, D = -0.51 cm(-1) = -0.73 K, and g = 2.00, where D is the axial zero-field splitting parameter. AC susceptibility measurements in the 1.8-7.0 K range in a zero DC field and a 3.5 G AC field oscillating at frequencies in the 50-997 Hz range revealed a frequency-dependent out-of-phase (chi(M)") signal below 3 K, indicating 2 to be a single-molecule magnet (SMM), the largest yet obtained. Magnetization versus DC field sweeps show hysteresis loops but no clear steps characteristic of quantum tunneling of magnetization (QTM). However, magnetization decay data below 1 K were collected and used to construct an Arrhenius plot that revealed temperature-independent relaxation below 0.3 K. The fit of the thermally activated region above similar to0.5 K gave U-eff/k = 15 K, where U-eff is the effective relaxation barrier. Resonant QTM was confirmed from the appearance of a "quantum hole" when the recent quantum hole digging method was employed. The combined results demonstrate that SMMs can be prepared that are significantly larger than any known to date and that this new, large Mn-30 complex still demonstrates quantum behavior.