Rabbit antibody to cutinase-I, isolated from Fusarium solani f. pisi, was conjugated to ferritin. With this ferritin-conjugated antibody it was shown that germinating spores of this fungus excreted cutinase during the penetration of the host pisum sativum. This result constitutes the most specific and strongest evidence for an enzymic penetration of a plant cuticle by a pathogen during infection.