Abstract: A method is provided for the isolation and presumptive diagnosis of Bacillus cereus which method is useful in the assay of antibiotic preparations, food products such as milk, drug products and cosmetics. A medium for use in such method is also provided which medium includes tellurite, glycine and polymyxin, and inhibits most organisms which grow and resemble Bacillus cereus on this medium. If an organism grows on the medium and has characteristic colonies, a presumptive diagnosis of Bacillus cereus can be assumed until further tests are made.