Inhibition of bacterial growth is the basis of the three methods commonly used for determining the potency of penicillin solutions. With the Oxford cup method (Abraham et al., 1941; Foster and Woodruff, 1944), penicillin is allowed to diffuse out into agar plates from small glass cups, and the diameters of the zones of inhibition of bacterial growth are measured. In dilution methods (Foster and Woodruff, 1943; Rammelkamp, 1942), the smallest amount of penicillin which will completely inhibit bacterial growth in solid or liquid media is the standard with which unknowns are compared. In turbidimetric methods, a range is selected in which bacterial growth is proportional to penicillin concentration, and a standard curve is constructed from turbidimetric measurements of the bacterial suspensions.