Plate glass sheets with hard wood or aluminum frames sterilized dry were used in place of petri dishes. A single layer of agar already inoculated with Bacillus subtilis was used, cavities made with a sterile cork borer replaced the usual porcelain cylinders, and the techniques of filling cavities and measuring zones of inhibition were standardized. The assay method finally adopted had fiducial limits of error of about 95-105% (P= 0.95).