A statistical method is descrd. to detn. the potency of antibiotic substances in terms of suitable standards and error of assay by use of a chart and a nomograph in conjunction with 4 figs. obtained by certain addns. and subtractions of the diams. of the zones of inhibition of incubated plates seeded with Staphylococcus aureus or other appropriate organism. The charts and methods can be used in similarly-designed assays of other drugs, e.g., in Vit. A assay.