Plasmid pAM28 obtained from Erwinia herbicola DNA suppresses the pathogenicity of E. carotovora. Its origin was confirmed by hybridization with E. herbicola DNA. The speA (suppressing pathogenicity of Erwinia) gene of pAM28 was localized. An open reading frame (ORF) coding for 166 amino acid residues and regulatory elements (promoter regions -10 and -35 and the Shine-Dalgarno sequence) were found. Mutant derivatives of pAM28 with deletions from the region of the SalI site did not suppress the pathogenicity of E. carotovora. Sequencing showed that a mutant plasmid pF16 identical to pAM28 in molecular weight and restriction sites but unable to suppress the pathogenicity of Erwinia had the speA identical to that of pAM28. This suggests that pAM28 carries at least two genes responsible for the suppression of pathogenicity.