The mycoparasite Trichoderma harzianum has been extensively used in the biocontrol of a wide range of phytopathogenic fungi. Hydrolytic enzymes secreted by the parasite have been directly implicated in the lysis of the host. Dual cultures of Trichoderma and a host, with and without contact, were used as means to study the mycoparasitic response in Trichoderma. Northern analysis showed high-level expression of genes encoding a proteinase (prb1) and an endochitinase (ech42) in dual cultures even if contact with the host was prevented by using cellophane membranes. Neither gene was induced during the interaction of Trichoderma with lectin-coated nylon fibres, which are known to induce hyphal coiling and appressorium formation. Thus, the signal involved in triggering the production of these hydrolytic enzymes by T. harzianum during the parasitic response is independent of the recognition mediated by this lectin-carbohydrate interaction. The results showed that induction of prb1 and ech42 is contact-independent, and a diffusible molecule produced by the host is the signal that triggers expression of both genes in vivo. Furthermore, a molecule that is resistant to heat and protease treatment, obtained from Rhizoctonia solani cell walls induces expression of both genes. Thus, this molecule is involved in the regulation of the expression of hydrolytic enzymes during mycoparasitism by T. harzianum.