ON Mar. 6, Prof. H. Wieland gave the second Pedler Lecture before the Chemical Society, his subject being “Recent Researches on Biological Oxidation”. Attempts have been made, he said, to formulate the vital process of combustion according to a single scheme. A ‘respiratory ferment’ catalysing the process is considered by Warburg to be related to hmin, and Liebig put forward the hypothesis that iron accelerates vital oxidations. If we adopt this view, wo are faced with the difficulty of understanding why the activated oxygen does not indiscriminately oxidise every substance which the cell presents to it. Yet if the process of biological hydrolysis is not carried out by any single ferment system, each kind of substrate being split by its own specific enzyme, it is unlikely that a single catalyst will suffice for the much more complicated process of the oxidative destruction of organic molecules.