A new method is described for determination of glutamic acid decarboxylase (GDC) activity. The measurement principle is based on the titrimetric determination of very small quantities of CO2 from the gas phase with the apparatus devised by Gorbach and Ehrenberger. The method was used to determine GDC activity in wheat, rye, and barley fumigated with Phostoxin pellets. Test readings did not show any differences between the activity of fumigated and non-fumigated material.