When carrying out hatching experiments it would frequently be an advantage to have a quantitative measure of the activity of the root excretion used and search was accordingly made for a stable substance which could be made to a solution of constant known strength and which could be used as the basis for the comparison of the activity of different samples of root excretion. Hurst (Hurst 1937) records that tyrosine in dilute solution, lysine in the form of its picrate as well as picric acid at ·01% concentration caused active hatching to occur and experiments using the above single cyst technique were conducted to investigate these claims. The results with the former two substances were disappointing but dilute picric acid solutions were found to be very active. Since it is hoped to publish a further paper in which the effects of picric acid will be considered quantitatively and in detail it is not proposed to publish data in this article but concentration experiments showed that picric acid is most active at ·02—·01% concentrations, the solutions being made up in distilled water. Several experiments were conducted in which picric acid in these concentrations was compared with water and with root excretion; in all cases picric acid hatches were significantly greater than water hatches although picric acid was not generally as active as root excretion taken from vigorously growing plants.