The binding of initiation factor IF-3 to Escherichia coli 30S ribosomal subunits has been found to be inhibited by rRNA ligands such as ethidium bromide, polyamines, and monovalent alkali metals. The order of effectiveness of the polyamines (spermine greater than spermidine greater than putrescine) and alkali metals (Li+ greater than Na+ greater than K+) in inhibiting the ribosomal binding of IF-3 parallels their degree of affinity for the RNA. Furthermore, the binding of IF-3 to 30S subunits chemically modified by photooxidation with rose bengal, nitration with tetranitromethane, and reaction with kethoxal, monoperphthalic acid, and p-chloromercuribenzoic acid was studied. Results obtained after the direct treatment of the 30S subunits with the above chemical reagents or upon reconstitution of 30S particles having a modified rRNA or ribosomal proteins indicate that the IF-3 binding site is preferentially lost when the rRNA becomes modified. It was found that IF-3 could bind normally to 30S subunits lacking protein S1 or proteins S11, S12, S19, and S21 (and perhaps S14) which had been cross-linked to IF-3 in other laboratories.