Trichoderma reesei is used by industry for production of plant material hydrolysing enzymes, especially cellulases. The fungus has excellent production and secretion capacity. The major cellulase cellobiohydrolase I (CBHI) represents half of the protein secreted and is encoded by a single copy gene. The strong cbh1 promoter and other promoters regulated in a different manner are available for protein production. The potential of the fungus in foreign protein production has been demonstrated by the expression of chymosin, interleukin-6, and laccase. Antibodies and their engineered forms have numerous applications. The capacity of Trichoderma to produce different forms of antibodies such as Fab molecules under the cbh1 promoter was analysed. When light chain was produced alone the secreted yields were very low but could be increased by introducing the heavy-Fd chain into the fungus. When the heavy-Fd chain was fused to the C-terminus of the CBHI core-linker region, production of secreted Fab's was increased about 50-fold. The amount of immunologically active CBHI – Fab molecules was about 150 mg/L in the medium in a fermenter cultivation. The released Fab molecules were authentic in their immunological properties demonstrating functional assembly of the light and heavy chains. The antibody part can be released from the CBHI fusion by an unidentified fungal protease or Kex2. The beneficial role of CBHI could be explained by more efficient transcription, ER entry or folding, or passage through the secretory pathway in general. Key words: heterologous expression, fusion protein, CBHI, Fab, single chain antibody.