The binding of tritiated tetracycline to Escherichia coli ribosomes has been investigated by a Millipore filtration technique. The effects of various conditions such as pH, temperature, and ionic strenght on this binding are reported. Apparently no covalent linkage is formed between the drug and the ribosomes. Evidence will be presented that such binding is due to electrostatic interactions between the drug and some component of the ribosome, possibly RNA. © 1969.