The fungal toxin associated with Dutch elm disease, cerato-ulmin, has been produced in the bacterium Escherichia coli by the assembly of oligonucleotides according to the unpublished amino acid sequence of the toxin. This toxin was produced at approximately 80 micrograms/L of cell culture as a fusion to glutathione S-transferase. We synthesized the toxin as a fusion protein to improve purification and stability. Recombinant cerato-ulmin was analyzed by immunoblot analysis and then separated from its fusion partner by thrombin. We incorporated this molecule into an appropriate medium to test the activity of the toxin on the growth of American elm callus cultures.