Diphtheria Virulence Agar Base is recommended for determining toxigenicity of Corynebacterium diphtheriae. C. diphtheriae is demonstrated by the formation of a white line of precipitate where it meets with diphtheria antitoxin diffusing from a strip of filter paper embedded in the agar. Potassium tellurite inhibits most gram-negative bacteria except Corynebacterium species, Streptococcus mitis, Streptococcus salivarius and Enterococci. Staphylococcus epidermidis may exhibit growth. False positive results may also be encountered. Therefore, a positive control has to always be run in parallel. Corynebacterium ulcerans and Corynebacterium pseudotuberculosis may also produce line of precipitation.