Abstract Pear cell-suspension cultures (PCSC) inoculated with pathogenic strains of Erwinia amylovora developed necrosis in 4–10 days. Necrotoxin preparations, produced by fractionation of necrotic PCSC, induced necrosis of pear seedling cuttings and inhibited the growth of PCSC. Fractionation of PCSC medium yielded a fraction that induced similar responses. Solutions of the inorganic salts used in the medium, similar in electrical conductivity to the necrotoxin preparations, affected pear seedling cuttings and PCSC in a manner that was indistinguishable from that of necrotoxin preparations. Toxic activity was retained following destruction of the organic compounds of a necrotoxin preparation by ashing. Upon fractionation of necrotoxin preparations by gel-permeation chromatography, all toxic activity migrated with the low molecular weight fractions that contained inorganic salts. Therefore, the toxic activity of the necrotoxin preparation was due to the inorganic salts from the tissue culture medium, concentrated to toxic levels by the purification procedures. No phytotoxin of bacterial origin was found in our preparations.