A molecular dynamics simulation study of the influence of a polymer melt matrix consisting of bead-necklace polymers on the effective interaction between two spherical nanoparticles was performed. The potential of mean force (POMF) between the two nanoparticles as well as entropy and energy contributions to the POMF was determined as a function of nanoparticle separation. The role of energy on the POMF was investigated by varying the strength of the polymer-nanoparticle interaction and comparing structure and POMF with those obtained for an athermal model. All features of the POMF as a function of nanoparticle separation were found to be strongly correlated with the polymer matrix density, the structure of the polymer at the nanoparticle interface, and the structure of the polymer in the interparticle region. The POMF was not found to correlate with polymer chain dimensions (e.g., radius of gyration) in contrast to colloidal suspensions in dilute and semidilute solutions. Both energy and entropy effects were found to make important contributions to the POMF. For the athermal system, where all matrix-induced interactions are entropic in nature, the nanoparticle POMF was found to exhibit qualitatively different behavior from that of the energetic systems. (C) 2003 American Institute of Physics.