Cycloheximide blocks the movement of peptidyl-tRNA from acceptor (aminoacyl) site to the donor (peptidyl) site on reticulocyte ribosomes. This translocation reaction is dependent upon the transfer enzyme, TF-II, and GTP hydrolysis. Cycloheximide has no effect on the ribosome dependent GTPase activity of TF-II or on the peptidyl transferase reaction by which peptides on tRNA in the donor ribosomal site are transferred to an amino acid on tRNA in the acceptor site. © 1969.