It was observed that during fermentative production of recombinant ovine interferon-tau (r-oIFN-tau) in Pichia pastoris, a secreted recombinant protein, the protein was degraded increasingly after 48 h of induction and the rate of degradation increased towards the end of fermentation at 72 h, when the fermentation was stopped. Proteases, whose primary source was the vacuoles, was found in increasing levels in the cytoplasm and in the fermentation broth after 48 h of induction and reached maximal values when the batch was completed at 72 h. Protease levels at various cell fractions as well as in the culture supernatant were lower when glycerol was used as the carbon source instead of methanol. It can be concluded that methanol metabolism along with cell lysis towards the end of fermentation contributes to increased proteolytic activity and eventual degradation of recombinant protein. (C) 2004 Wiley Periodicals, Inc.