With isolated 30S ribosomal subunits, binding of T4 mRNA is dependent on initiation factors in the absence of formyl Met-tRNAf. Formation of the 70S-mRNA complex, however, occurs only when formyl Met-tRNA and GTP are added. Initiation factors therefore control directly 30S-mRNA interaction independent of their effect on the binding of initiator tRNA. © 1969 Nature Publishing Group.