  We present results of a lattice computation of the vector and axial-vector
current matrix elements relevant for the semileptonic decay B^0-bar -> rho^+
l^- nu_l-bar. The computations are performed in the quenched approximation of
lattice QCD on a 24^3 x 48 lattice at beta = 6.2, using an O(a) improved
fermionic action. Our principal result is for the differential decay rate,
dGamma/dq^2, for the decay B^0-bar -> rho^+ l^- nu_l-bar in a region beyond the
charm threshold, allowing a model-independent extraction of |V_{ub}| from
experimental measurements. Heavy quark symmetry relations between radiative and
semileptonic decays of B-bar mesons into light vector mesons are also
discussed.
