The presence or absence of residues of an antibiotic such as penicillin is rapidly determined in a sample such as milk or meat. The sample is added to a test vessel containing a solidified agar medium containing spores of a microorganism which has high sensitivity for the antibiotic being determined. After addition of nutrients and incubation, presence or absence of the antibiotic is indicated by the extent of growth of the microorganism. The nutrients may be in the form of a tablet which may be placed on the surface of the solidified agar medium prior to use. The tablet may be coated with a layer preventing moisture transport from the medium into the tablet during storage, but allowing nutrient transport under test conditions. The extent of microorganism growth may be determined visually or indicated by an indicator present in the solidified agar medium or the nutrient tablet. The test vessel preferably has a cross-section of 3 to 20 mm and a height of 3 to 30 mm.