On the nature of the pseudo-hybrid formed, at room temperature, from denatured DNA and ribosomal RNA. - It has been shown that, at room temperature, the biologically active strand of phage $\alpha$, active on B. magatherium, binds segments of ribosomal RNA from a non-infected host. In the light of this result, the biological significance of such pseudo-hybrids is discussed, and it is pointed out how, in the phage-host cell system, their existence can give rise to difficulties in interpreting those experiments in which an attempt is made to identify, in the infected bacterium, the phage messenger RNA by means of its capacity to bind the denatured DNA of the phage itself.