PENICILLIN I. METHODS OF ASSAY1

Investigations involving the evaluation of bactericidal and bacteriostatic substances are dependent upon rapid and reliable assay methods. The cup plate method and the serial dilution method are the two procedures most commonly employed. Techniques for assaying penicillin have been developed by Foster (1942), Rammelkamp (1942), and Abraham, Chain, et al. (1941), the latter having devised the cylinder plate procedure which is a modification of the cup plate method.. This modified method has been used extensively by the authors during recent months, during which time several pertinent observations and refinements have resulted. It is the purpose of this paper to present these features, as well as to describe a serial dilution procedure which has also been found useful in penicillin assaying. II. CYLINDER PLATE METHOD The cylinder plate procedure can only be used to test those substances which readily diffuse through agar. The method is a modification of the well-known cup plate method, yet it is unique and different in that glass or porcelain cyl