The initiation of protein synthesis, in prokaryotes and eukaryotes, requires the participation of ribosomal subunits, a specific initiator aminoacyl-tRNA molecule, and specific protein initiation factors that bring about the interaction between ribonucleoprotein particles, protein factors, and various nucleic acids. Possible intermediates in the ribosome cycle, between the termination of protein synthesis on a given cistron and the initiation of another, are ribosomal subunits in association with a number of proteins required for initiation. Some of the activities on native or free ribosomal subunits have been identified in abstracts prepared from such particles obtained from rat liver cytoplasm, by extractions with solutions containing high concentrations of KCL. One of these activities, purified in this laboratory, is a high molecular weight protein (500,000-700,000) that partially dissociates 80S ribosomes to subunits and completely prevents the reassociation of ribosomal subunits to ribosomes. Two distinct activities, previously identified in a number of eukaryotic cells, catalyze the binding of initiator Met-tRNAf to 40S subunits; one of these requires template and magnesium ions, but not GTP, while the other requires GTP but not template or Mg ions. Both of these Met-tRNAf binding activities have also been obtained in the high-salt extract of native 40S ribosomal subunit. Purification of the proteins, biological and physical-chemical characterization of the proteins, and studies on the mechanism of action in the binding reaction(s) that they catalyze are in progress.