Abstract: A method for the rapid determination of the presence or absence of residues of antibiotic such as penicillin, in liquids, for example milk, and meat comprising introducing spores of a microorganism which possesses a high sensitivity for the antibiotic to be determined into an agar medium so that the spores are prevented from starting to germ due to lack of nutrients, but stay alive, and being sufficiently heavily inoculated that after addition of nutrients and incubation at or near optimal temperature in a short time sufficient growth has proceeded so that growth is either observable visually or its indicated by an indicator added to the agar medium, the said spore culture being allowed to solidify in upright test vessels having a cross-sectional dimension sufficiently small to require as little of the agar medium as possible but sufficiently large to enable a correct determination of inhibition of growth of the microorganism to be made, and having a height so that the vessel can contain a sufficient amount o