The interaction of wheat germ eIF-3 with the wheat germ cap-binding proteins eIF-(iso)4F and eIF-4F as a function of pH and ionic strength is described. Direct fluorescence titration experiments are used to measure the equilibrium association constants (Keq) for the binary protein/protein complexes as well as for the interaction of eIF-3 with methylated cap analogues and rabbit alpha-globin mRNA oligonucleotide analogues. The Keq values for ternary eIF-3/eIF-(iso)4F/analogue and eIF-3/eIF-4F/analogue interactions were also measured. The equilibrium binding constants were used to calculate coupling free energies, which provide an estimate of the cooperativity for the interaction of the mRNA analogues, eIF-3, and either eIF-4F or eIF-(iso)4F. These data suggest a mechanism in which the binding of eIF-(iso)4F or eIF-4F to mRNA enhances the subsequent binding of eIF-3 to the message. This may lead to favorable positioning of the complex on the ribosome and thereby enhance translation.