Bacterial ribosomal RNA and rapidly labelled RNA interact in the presence of sufficient concentrations of monovalent (0·6 M), or divalent (0·01 M) cations to form complexes. The formation and properties of these complexes have been studied using the techniques of sedimentation on sucrose gradients, and chromatography on Hershey columns. It is shown that the separation of the rapidly labelled component of bacterial RNA into three fractions by chromatography on Hershey columns is due to complex formation between this material and ribosomal RNA during chromatography. Rapidly labelled RNA forms complexes with both subunits (16 s and 23 s) of ribosomal RNA. This association is dependent upon temperature and pH, being observed only at temperatures lower than approximately 15°C, and at pH values lower than 9·5. High concentrations of formamide, a nucleic acid denaturing agent, or of four protein synthesis inhibitors (streptomycin, chloramphenicol, actinomycin and puromycin) do not interfere with complex formation. Heat treatment of ribosomal RNA or of purified rapidly labelled RNA (free of ribosomal RNA) shows that interaction persists even after considerable thermal degradation of either of these materials has occurred. Association of rapidly labelled and ribosomal RNA is probably caused by interactions between complementary bases in the interacting polynucleotide chains. Although this phenomenon resembles in several respects the interaction of messenger RNA with ribosomes, there are important differences between the two processes.