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Introduction: Potentially toxigenic Corynebacteria (Corynebacterium diphtheriae, Corynebacterium ulcerans, and Corynebacterium pseudotuberculosis) can produce diphtheria toxin and stated as diphtheria causative agent. The bacteria causing diphtheria could be identified by Cystinase test on the Tinsdale medium, while its toxigenicity determined by Elek test on the Elek medium. This study aims to develop a new modified medium for both Cystinase and Elek tests simultaneously.Methods: There were ten reference strains of bacteria used for the modified medium optimization. Moreover, 15 clinical isolates were used as samples in the modified medium testing. The result of Cystinase and Elek tests on the modified medium was compared with the standardized tests on the Tinsdale and Elek mediums.Results: Twelve of 25 isolates tested on the modified medium were identified as toxigenic strain, corresponding with the result from standardized Elek test on the Elek medium. Moreover, 16 of 25 isolates tested on the modified medium were identified as positive for Cystinase test. The similar result was obtained using the standardized Cystinase test on the Tinsdale medium. This result was visible 24 hours after incubation. The modified medium was in excellent condition with the consistent result after stored in half-finished condition for 32 days at 2-8 oC.Conclusion: The modified medium developed in this study was a new good medium that could be used for Cystinase and toxigenicity tests simultaneously.