Ribosomal groups essential for messenger binding are tentatively identified by the effects of simple chemical pretreatments on ribosome binding activity. Ribosomes treated with formaldehyde rapidly lose their ability to bind polynucleotides. Formaldehyde inactivation results from direct attack on amino groups in the binding site. Nitrous acid also inactivates ribosomes. Dinitrofluorobenzene, an amine reagent which reacts only with ribosomal protein, has no effect on the capacity of ribosomes to bind polynucleotides. It does, however, prevent the messenger-directed binding of sRNA. Perphthalic acid, which selectively oxidizes nucleotides, also inhibits messenger attachment. The association of 30 s and 50 s ribosomal subunits to form the 70 s particle is affected by these reagents in the same way as mRNA binding. Apparently ribosomal RNA amino groups are vital for both functions. It is proposed that messenger binding results from hydrogen-bond formation between ribosomal RNA amino groups and messenger phosphates. The data are consistent with either a base-base or a base-phosphate hydrogen-bonding mechanism for the 30 s–50 s aggregation.