In the present work, experiments were carried out to determine the possible mechanisms of action of isolates of Trichoderma spp as a biocontrol agent of leaf blotch of wheat. The Trichoderma spp. isolates were coming from soil cultivated with wheat in La Plata area. Thirty isolates were characterized. For the greenhouse bioassay the following treatments were done: control plants, plants inoculated with S. trítici, wheat seeds pelleted with Trichoderma spp. and wheat seed pelleted with Trichoderma spp and inoculated with S. trítici. After 21 days of inoculation, in the first leaf the necrotic area and pycnidial coverage were evaluated. The leaf apoplast antifungal proteolytic activity and dry weight of aerial parts and roots was measured in plants treated with the strains that demonstrated the best behaviour as biocontrol agents. The results showed that the strains 2, 4, 5 y 8 decreased significantly tne necrotic area en the pycnidial coverage. Thus, this strains have biocontrol activity against the disease. The mechanism of action of those strains were investigated. Plants grown from seed coated with 8 and 2 isolates of Trichoderma spp. demostrated an increased antifungal proteolytic activity, it could be related with sistemic induced resistance in plants. On the other hand, The strains 4 and 5 they produced an increase of dry weight of roots and aerial parts respectively producing plant more robust and as a consequence the disease reduction.