ABSTRACT There are several ribosomal proteins which can bind specifically to the three ribosomal RNA's of E. coli. Some structural properties of these proteins and of the RNA are described together with the optimal conditions for binding. Evidence for conformational changes occurring in some proteins, and in the RNA's during binding, is also discussed. The partial localisation of protein binding sites on the 16S and 5S RNA's, and for one protein on the 23S RNA has been attained recently. However, relatively little is known about the regions of the protein which interact with the RNA. Although the nature of the specificity of the protein-RNA interactions is not yet understood, some experimental approaches which are in progress to elucidate the basis of this specificity are mentioned together with a discussion of the structural factors which may be important.