How faithful are N-body simulations of disc galaxies?

High-softening two-dimensional models, frequently employed in N-body experiments, do not provide faithful simulations of real galactic discs. A prescription [Eqs. (17) and (18)] is given for choosing meaningful values of the softening length in standard regimes of astrophysical interest, when both the stellar and gaseous components are present. In addition, a stability criterion [Eq. (10); see also Eq. (11)] is given for choosing meaningful input values of the Toomre parameter for a given softening length. Such a criterion should also provide a key to the correct interpretation of computational results in terms of real phenomena.