We study the effect of a magnetic field (applied along the c axis) on the low-energy, incommensurate magnetic fluctuations in superconducting La1.82Sr0.18CuO4. The incommensurate peaks at 9 meV, which in zero field were previously shown to sharpen in q on cooling below T-c [T. E. Mason , Phys. Rev. Lett. 77, 1604 (1996)], are found to broaden in q when a field of 10 T is applied. The applied field also causes scattered intensity to shift into the spin gap. We point out that the response at 9 meV, though occurring at incommensurate wave vectors, is comparable to the commensurate magnetic resonance observed at higher energies in other cuprate superconductors.