The effective interaction in nuclear matter ( G-matrix) is calculated, in the relativistic (Dirac-Brueckner-Hartree-Fock) as well as in the conventional Brueckner-Hartree-Fock approach. Starting point is a nucleon-nucleon potential which, in addition to single-meson exchange, contains delta-isobar box diagrams with ππ- and πϱ-exchange, leading to Pauli blocking and dispersive effects. The implications for nuclear matter binding and Landau parameters are investigated.