MANY organophosphorus insecticides are poor inhibitors of cholinesterase, but are converted to potent inhibitors (‘activated’) by certain mammalian and insect tissues. The activation by liver slices was first shown by Gardiner and Kilby1 for schradan (a phosphoroamidate), and by Diggle and Gage2 for parathion (a phosphorothionate). Davison3 showed that both these activations could be effected by the combined soluble and microsomal fraction of liver, if magnesium, nicotinamide and diphosphopyridine nucleotide were provided. I have shown that the soluble fraction could be replaced by reduced diphosphopyridine nucleotide for activation of schradan4 and malathion5. In the present work, a slightly different technique was used for demonstrating activation of phosphorothionate by liver microsomes with reduced diphosphopyridine nucleotide.