1. Pyrethrum powders and dusts, prepared by grinding or by the incorporation of extracts of pyrethrum flowers upon absorbent earths, such as talc and kieselguhr, lose their insecticidal activity on exposure to light and air. The loss is more rapid in the case of artificially prepared dusts than with ground flower-heads.

2. Both light and air play an important part in the process of inactivation, as samples of kieselguhr-pyrethrum and talc-pyrethrum dusts stored in closed vessels in the dark or exposed to air in the dark are relatively stable; also samples exposed to light in an atmosphere of carbon dioxide, nitrogen or in vacuo lose little of their toxicity under the same conditions of illumination; samples exposed in oxygen, however, rapidly lose their activity.

3. Both wet and dry oxygen were effective in destroying the activity of the dusts, but apparently at different rates, and there is some suggestion that the type of reaction may be different in the two cases.