The resistance of some penicillins to beta-lactamase enzymes was previously attributed to the nature of their C(6) side chain. In order to find explicitly the influence of the conformation of this side chain in the enzymatic mechanism, we have analyzed by experimental and theoretical methods (X-ray diffraction, NMR, IR, PCILO) the molecular structure of six resistant penicillins and derivatives: oxacillin, cloxacillin, dicloxacillin, flucloxacillin, methicillin, nafcillin, cloxacillin sulfoxide, and oxacillinpenicilloic acid. X-ray crystallography of flucloxacillin and nafcillin is fully described. We observe that the side chains of these penicillins have no influence on the electronic properties of the penam nucleus but are much more rigid than in the sensitive ones. The molecular conformations are mostly governed by the nonbonded Van der Waals interactions and, in the oxacillins, partly by the conjugation between exocyclic groups. The lack of flexibility could result in a distorting effect on the structure of the active site of the beta-lactamase, leading to the deactivation of the enzyme.