The antibiotic zone reader provides a rapid accurate method for determining the strength of antibiotic materials by measuring the diameter of an inhibition zone in a petri-dish. This zone reader measures the diameter of the inhibition zone to 0.02 mm within range of 0 to 35 mm diameter. Light from a source from the top, passes through the transparent and semitransparent portions of the agar, then to a reflecting mirror supported by an arm above the unit. The mirror reflects the light to glass prism mounted at the front of the unit and magnified image of the zone of inhibition is clearly visible on the prism.