Using diethyl pyrocarbonate as enzyme inhibitor, RNA was extracted from embryos of germinating barley kernels. The template activity of this RNA was tested by its capacity to stimulate [14C]phenylalanine incorporation into trichloroacetic acid insoluble protein synthesized in an Escherichia coli cell free system. The template activity was compared with that of RNA isolated from barley embryos by other methods of extraction. The RNA extracted by the ðyl pyrocarbonate method, omitting sodium dodecyl sulfate, was found to have better template activity than the others, and was equivalent or superior to tobacco mosaic virus RNA isolated by the phenol method. Sucrose density gradient sedimentation of the RNA yielded fractions with still higher specific template activity. Reextraction with phenol of the RNA extracted by the diethyl pyrocarbonate method reduced the template activity of the RNA. Native and denatured DNA were also found to decrease the template activity of RNA. RNA extracted from kernels germinated in the presence of 5-fluorodeoxyuridine had a reduced template activity indicating that this compound may inhibit mRNA synthesis, possibly indirectly by blocking strand separation in DNA synthesis. [7]. The results obtained are however confusing and evade critical comparison mainly because (a) the cell free protein synthesising systems and the experimental conditions employed by various authors are different; (b) the procedures of isolation of mRNA are different although in most of the cases the extraction of mRNA was done by various versions of the "hot phenol" method [S], a procedure known to lead to aggregation and partial degradation of RNA [9] ; in addition ribonuclease activity may survive treatment by phenol [lo, 111 which implies an isolation of RNA and especially mRNA degraded to M e r e n t extents; and (c) in most of the experiments no proper standards (such as poly U, or viral RNA) were used; hence they do not permit an evaluation of the quantity of mRNA isolated.Unusual Abbreviations. FUdR, 5-fluorodeoxyuridine; mRNA, messenger RNA; poly U, polyuridylic acid; TMV, tobacco mosaic virus; tRNA, transfer RNA.The greatest obstacle in the isolation of mRNA is its degradation by the ribonuclease [12] during extraction. (According to our observations even bentonite, which is certainly not a specific adsorbent for ribonuclease, is ineffective in completely inhibiting this effect.) A method for the isolation of undegraded nucleic acids, utilizing diethyl pyrocarbonate as a protein denaturing general enzyme inhibitor [13,14] has been described earlier [I51 and it has also been shown that diethyl pyrocarbonate inhibits deoxyribonuclease and ribonuclease without affecting the substrate properties of the nucleic acids [16] and bacterial DNA treated with diethyl pyrocarbonate retains its biological activity [17]. Under the conditions used for the extraction of tissue RNA, diethyl pyrocarbonate leaves transfer RNA (tRNA) functionally intact [18]. I n the present paper we describe attempts to use diethyl. pyroc...