The diagnostic characteristics of 8 nutrient media most frequently employed at practical bacteriologic laboratories for the isolation of C. diphtheriae have been under study. Clauberg-2 medium and blood-tellurite agar have been found the most suitable for the diagnostic studies. Addition of extra ingredients to the standard recipe deteriorates the diagnostic potentialities of the media. Tinsdal's medium is characterized by the lowest diagnostic efficacy. The schemes of using these media are presented, with due consideration for their diagnostic potentialities.