Proteins that are located adjacent to the 5' end of mRNA in initiation complexes have been detected by chemical crosslinking. Reovirus mRNA containing radioactivity exclusively in the [3H]methyl-labeled "cap," m7G(5')ppp(5')-Gm, was oxidized with sodium periodate to convert the 2',3'-cis-diol of the 5'-terminal m7G to a reactive dialdehyde. Oxidized mRNA was incubated in cell-free protein-synthesizing systems derived from wheat germ or mammalian cells, and the resulting mRNA-ribosome initiation complexes were reduced with NaBH3CN. By this chemical procedure, putative Schiff bases between mRNA 5'termini and amino groups of neighboring proteins were stabilized by reduction, yielding covalently linked protein-RNA conjugates. Under conditions of ribosome binding, a limited number of polypeptides associated with the mRNA-ribosome complexes were crosslinked, suggesting that these proteins are positioned near and may interact with the 5' end of mRNA during initiation. This method should also be useful for studying the spatial relationships between molecules in other similar nucleoprotein complexes.