Mature green tomatoes immediately after harvest were resistant to infection by Botrytis cinerea, although if held for 2 days at 20°C they lost this resistance. Heat treatment (38°C for 3 days) after inoculation prevented B. cinerea development. It inoculated following the heat treatment lesions developed but more slowly than on control fruit. Only 60°0 of the heated fruit developed lesions if inoculated after 3 days at 38°C and the average size was 1.6 cm, while 100°0 of the control fruit developed decay and the average decay diameter was 3.5 cm. A highly anionic peroxidase (TAP) mRNA was abundant in mature green tomatoes at harvest and decreased rapidly as the fruits were held at 20°C. Holding fruit at 38°C prevented the decrease in TAP mRNA, but once fruits were transferred to 20°C the loss of mRNA was rapid. Soluble peroxidase activity decreased as harvested fruits were held at 20°C, while during heat treatment activity was higher than at harvest. NaCl-extractable peroxidase activity was higher in fruits held at 20°C or 38°C than at harvest, but a greater increase was seen in heated fruits. Activity gels showed the appearance of new isoformes in heated tomatoes. We suggest that peroxidases may be involved in the resistance of heated tomatoes to pathogen infection.