Inhibition of aminoacyl-tRNA binding by the purified initiation factors f1 and f2 could be regarded as a mechanism for ensuring that only F-met-tRNA binds initially to the entry site, thereby preventing random or non-specific binding by aminoacyl-tRNA. This inhibition would be relieved by the hydrolysis of the GTP molecule normally involved in the binding of F-met-tRNA. © 1968 Nature Publishing Group.