We have evaluated the total carrier mass enhancement factor f_{t} for MgB_{2} from two independent experiments (specific heat and upper critical field). These experiments consistently show that f_{t} = 3.1pm0.1. The unusually large f_{t} is incompatible with the measured reduced gap (2Delta (0)/k_{B}T_{c} = 4.1) and the total isotope-effect exponent (alpha = 0.28pm0.04) within the conventional phonon-mediated model. We propose an unconventional phonon-mediated mechanism, which is able to quantitatively explain the values of T_{c}, f_{t}, alpha, and the reduced energy gap in a consistent way.
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