We have calculated the resonant-tunneling conductance for two-dimensional samples. For a disordered system, most of the resonant peaks in the conductance are Lorentzians with height less than 1 (in units of ${e}^{2}$/h), but in some special cases two resonant peaks can be very close and overlap, and the maximum conductance can be larger than 1. In a magnetic field, the height and width of the resonant peak fluctuate when the magnetic flux through the area of the localized state changes by roughly unity. The fluctuation in the peak height is the same order as the quantity itself. There is some evidence that the average localization length may increase slightly with the magnetic field.