A recombinant human consensus interferon-alpha mutant (cIFN) was expressed in Pichia pastoris. The maximum dry cell weight, cIFN concentration and antiviral activity were 160 g l(-1), 1.24 g l(-1) and 4.1 x10(7) IU ml(-1)s, respectively. The cIFN secreted into the medium was in the form of aggregates dominantly by non-covalent interaction and partially by disulphide bond. When the fermentation supernatant was disaggregated with 6 M guanidine hydrochloride, the antiviral activity of cIFN achieved 2.2 x 10(8) IU ml(-1).