Microbiological assay of vitamins is a type of. Biological assay performed with the aid of microorganisms. Many therapeutic agents, which either inhibit the. Growth of microorganisms or are essential for the growth of them are standardized by microbial assay. The microbiological assay is based upon a comparison of the inhibition of growth of micro-organisms by measured concentration of the antibiotics to be examined with that produced by known concentrations of a standard preparation of the antibiotic having a known activity.