The effects of ethidium bromide, an intercalating dye and berenil, a nonintercalating dye on the biological activities ofEscherichia coli ribosomes have been studied. Ethidium bromide treatment drastically reduced both enzymatic and nonenzymatic initiation complex formation, enzymatic as well as nonenzymatic binding of phenylalanyl tRNA, peptidyl transferase, GTPase as well as the overall protein synthesising activity as measured by the poly U-dependent polymerization of phenylalanine. On berenil treatment, however, only enzymatic formation of the initiation complex is marginally reduced. Other reactions are not markedly affected except the enzymatic phenylalanyl tRNA binding which is slightly decreased only at high Mg2+ concentration; the treated ribosome has lowered polymerizing activity at sub-optimal Mg2+ concentration (10 mM). Although it has already been shown in this laboratory that treatment with either dye leads to the unfolding of the structure of the ribosome, the present studies indicate that berenil treatment does not alter the structure of the ribosome drastically in contrast to ethidium bromide treatment.