The effects of the heat stable substance excreted by a virulent strain of Pseudomonas solanacearum on tissue cultures of tomatoes were studied.The results showed that there existed a kind of toxic component in the plateoculture filtrate(PCF),which was demonstrated to be neither enzymes nor extracellular polysaccharide (EPS). This substance was toxic to the differentiation of tomato leaf discs and the rooting of shoots,while EPS had not such effect.