The magnetic field dependence of the spin-susceptibility, $chi_{s}$ was measured in the superconducting state of high purity MgB$_{2}$ fine powders below 1.3 T. $chi_{s}$ was determined from the intensity of the conduction electron spin resonance spectra at 3.8, 9.4, and 35 GHz. At the lowest magnetic fields (0.14 T), a gap opens in the density of states at the Fermi energy and, accordingly, $chi_{s}(T)$ is small at low temperatures. Fields above 0.2 T (about 15 % of $H^{c}_{c2}$, the minimum upper critical field), destroy the gap. The field induced $chi_{s}$ is much larger than expected from current superconductor models of MgB$_{2}$.
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