Elek's method has been generally used to examine the toxinogenicity of Corynebacterium diphtheriae in vitro, but the results of tests were not always satisfactory. We succeeded to produce a peptone suitable for toxin production of C. diphtheriae. We also found that papain digests of sera promoted greatly the growth of the organisms on agar media in the absence of serum. These observations made us possible to prepare a new medium for Elek's test. This medium consists of: 1.2% digested sera, 2.5% peptone (papain digest of the whale meat), 0.2% yeast extract, 1.0% maltose, 0.1% NaCl and 1.5% agar. The pH is adjusted to 7.6 and the medium is sterilized for 20 minutes at 120°C. The medium can be stored in a powdered form. The elimination of the serum facilitated the preparation and sterilization of the medium. It was also demonstrated that the antitoxin could be stored in a dried state on the filter paper.