Double-strand formation occurs with high molecular weight DNA embedded in agar on incubation with either messenger RNA or sheared, denatured DNA. Both the DNA in the agar and the DNA fragments must be previously denatured for reaction to occur. The fraction of the total RNA in E. coli represented by the messenger type is about 1·3%. Essentially all of the DNA trapped in the agar can form duplex structures with DNA and about half with RNA. The latter result implies that there exist in a cell population RNA molecules homologous to half of the possible sequences in the DNA. In view of the inability of purified messenger RNA molecules to form double-stranded structures when incubated alone, the interpretation has been made that RNA molecules are complementary to one strand of the DNA and identical to the other. Thus, essentially all of the genome is active in making RNA, although from the nature of the saturation curve of DNA with RNA it would appear that the relative abundances of the various RNA molecules are widely different.