Cystine Tellurite Agar Base is recommended for selective isolation and differentiation of Corynebacterium diphtheriae types. Cystine Tellurite Agar Base with the addition of sterile sheep blood is used for selective isolation and differentiation of Corynebacterium diphtheriae types. This medium is differential on the basis of the ability of Corynebacterium species to reduce tellurite whereas diphtheroids found in upper respiratory tract are not able to reduce tellurite. L-Cystine is the source of amino acid, which enhances H2S production. Further biochemical tests are necessary to distinguish between C.diphtheriae and C.ulcerans due to similar reactions on this medium.