New results on nonadditive effects on small beryllium clusters are presented using the LCAO-MO-SCF method including variational and perturbational (MP-4) configuration effects. Three-and four-body contributions to the interaction energy of these clusters are dominant, and are used to explain the very different behaviour of bulk beryllium as compared to liquid helium. This is relevant because the atomic ground states of Be and He are strikingly similar (ls'2s2: IS, and 1s' : IS,) and also HeZ and Be2 share the honour of being the two hardest-to-detect dimers in the whole periodic system. The similitude is lost for larger He,, and Be,, systems because the former, as was shown by R.F.W. Bader 25 years ago, have very small three-body energies ( 5 1%). Be,, clusters will be shown here to have three-body energies that actually outweigh the painvise additive ones. The consequences of this are discussed.Key words: intermolecular forces, non-additivity, He and Be clusters. Mots clCs : forces intermolCculaires, non-additivitk, agrCgats de He et Be.[Traduit par la redaction]