A no. of promising synthetic catalysts for the hydrolytic degrdn. of RNA have been developed in recent years. Some of them show remarkable selectivity for pyrimidine nucleotides. The general problem of all these studies is to distinguish between real effects and artifacts caused by traces of contaminating natural RNases. We show that methods representing the current state of the art (diethylpyrocarbonate treatment, sterilization, ultrafiltration, etc.) do not sufficiently protect against severe artifacts. However, an incorruptible assay could be found by comparing the cleavage of RNA and its mirror image. Enantiomeric RNA is completely resistant to enzymic degrdn., whereas achiral nonpeptide catalysts, by fundamental laws of symmetry, cannot distinguish between enantiomers and will induce exactly the same cleavage pattern with both substrates. [on SciFinder (R)]