THE prediction of a protein's tertiary structure is still a considerable problem because the huge amount of possible conformational space' makes it computationally difficult. With regard to side-chain modelling, a solution has been attempted by the grouping of side-chain conformations into representative sets of rotamers 2-5. Nonetheless, an exhaustive combinatorial search is still limited to carefully identified packing units 5,6 containing a limited number of residues. For larger systems other strategies had to be developed, such as the Monte Carlo Procedure 6,7 and the genetic algorithm and clustering approach 8. Here we present a theorem, referred to as the 'dead-end elimination' theorem, which imposes a suitable condition to identify rotamers that cannot be members of the global minimum energy conformation. Application of this theorem effectively controls the computational explosion of the rotamer combinatorial problem, thereby allowing the determination of the global minimum energy conformation of a large collection of side chains.