Direct numerical simulations of a turbulent fluid laden with finite-sized particles are performed. The computations, on a 1283 grid along with a maximum of 262 144 particles, incorporated both direct particle interactions via hard-sphere collisions and particle feedback. The ‘reverse’ coupling is accomplished in a manner ensuring correct discrete energy conservation (Sundaram & Collins 1996). A novel two-field formalism (Sundaram & Collins 1994a) is employed to calculate two-point correlations and equivalent spectral densities. An important consideration in these simulations is the initial state of fluid and particles. That is, the initial conditions must be chosen so as to allow a meaningful comparison of the different runs. Using such a carefully initialized set of runs, particle inertia was observed to increase both the viscous and drag dissipations; however, simultaneously, it also caused particle velocities to correlate for longer distances. The combination of effects suggests a mechanism for turbulence enhancement or suppression that depends on the parameter values. Like previous investigators, ‘pivoting’ or crossover of the fluid energy spectra was observed. A possible new scaling for this phenomenon is suggested. Furthermore, investigations of the influence of particle mass and number densities on turbulence modulation are also carried out.