We calculate the Isgur-Wise function by measuring the elastic scattering amplitude of a D meson in the quenched approximation on a 24 3 48 lattice at = 6:2, using an O(a)-improved fermion action. Fitting the resulting chirally-extrapolated Isgur-Wise function to Stech's relativistic-oscillator parametrization, we obtain a slope parameter 2 = 1:2 + 7 3 . We then use this result, in conjunction with heavy-quark symmetry, to extract V cb from the experimentally measured B ! D l di erential decay width. We nd jV cb j p B =1:48ps = 0:038 + 2 2 + 8 3 , where the rst set of errors is due to experimental uncertainties, while the second is due to the uncertainty in our lattice determination of 2 .Typeset using REVT E X 1