VARIOUS substances activate certain insecticides, especially the pyrethrins; that is to say, these substances, though themselves non-toxic, intensify the toxic action of pyrethrins or other insecticides upon insects. However, no attempt seems to have been made up to now to compare activation effects quantitatively. Many methylenedioxyphenyl compounds are good activators, and two preparations containing them have been investigated, namely, (A) one containing 80 per cent of (3,4-methylenedioxy-6-propyl benzyl) (butyl) diethylene glycol ether and 20 per cent of related compounds, and (B) one containing the n-octyl sulphoxide of isosafrole. Both these materials were found to activate pyrethrins, allethrin, and γ-benzene hexachloride (BHC) against flour beetles, Tribolium castaneum1. The relationship between mortality and the concentrations of insecticide and activator has been investigated, using the materials in solution in Shell oil P31 (with benzene as an intermediate solvent where necessary): the solutions were applied at fixed deposit by means of a direct spray technique2. In addition, an effort has been made to compare quantitatively the intensities with which the two activators activate each insecticide, and with which the different insecticides are activated by each activator.