Leaf protoplasts from near-isogenic tomato (Lycopersicon esculentum) cultivars, differing only at the I 1 resistance gene, showing differential cell death in response to culture filtrates of Fusarium oxysporum f.sp. lycopersici race 1 were used to quantify the toxic activity of crude and partially purified culture filtrates. Toxic activity was partially lost after treatment with protease or storage at pH values above neutrality and was completely destroyed by autoclaving. Fractions with toxic activity were separated by gel filtration. Following acid native polyacrylamide gel electrophoresis and electro-elution of a desalted fraction precipitated by 20% saturation with ammonium sulphate from crude culture filtrates, toxic activity was found to be associated with a single proteinaceous band with an R F value of 0.32. Electrophoresis under denaturing conditions revealed that the toxin was comprised of two bands, with apparent molecular weights 56 and 61 kDa, which stained positively for protein. Protoplasts of cv. Ace, lacking resistance genes, were sensitive to the toxin, with a LD 5 0 of 55 ng protein ml - 1 compared with >22 μg protein ml - 1 against cv. Royal Ace protoplasts, possessing the I 1 resistance gene to F. oxysporum f.sp. lycopersici race 1. Injection of the toxin into tomato plants induced symptoms similar to those of natural infection in susceptible cv. Ace, but caused no symptom development in resistant cv. Royal Ace. This corresponded to the cultivar race interactions observed under field conditions.