The eukaryotic translation initiation factor 2 (eIF2) is pivotal for delivery of the initiator tRNA (tRNAi) to the ribosome. Here, we report the functional characterization of the archaeal homologue, a/eIF2. We have cloned the genes encoding the three subunits of a/eIF2 from the thermophilic archaeon Sulfolobus solfataricus, and have assayed the activities of the purified recombinant proteins in vitro. We demonstrate that the trimeric factor reconstituted from the recombinant polypeptides has properties similar to those of its eukaryal homologue: it interacts with GTP and Met-tRNAi, and stimulates binding of the latter to the small ribosomal subunit. However, the archaeal protein differs in some functional aspects from its eukaryal counterpart. In contrast to eIF2, a/eIF2 has similar affinities for GDP and GTP, and the β-subunit does not contribute to tRNAi binding. The detailed analysis of the complete trimer and of its isolated subunits is discussed in light of the evolutionary history of the eIF2-like proteins.