A method is described for separating fungal and host pectinmethylesterase from tomato plants infected with Fusarium oxysporum f. lycopersici. The results obtained, supported by studies on the effect of detergent and pH on the activity of enzyme from infected plants, show that, at least at later stages of infection, there is a large increase in host enzyme activity and that fungal pectinmethylesterase is less than 10% of the total.