Abstract: A fixed functional relationship in either tabular or equation form is established between the growth of various microorganisms in the presence of a few (e.g., one or two) predetermined concentrations of a predetermined antibiotic and the minimum concentration of such antibiotic necessary to at least inhibit the activity of such samples. The relationships are established for each desired combination of antibiotic and general class of microorganisms and the degree of growth is measured at a predetermined time or level of growth, preferably before saturation occurs at "growth" or "no growth" extremes. The minimum concentrations used in deriving these fixed relationships are determined by standard accepted quantitative techniques.