Expression of human tumor necrosis factor —α (TNF) and four different TNF analogs has been studied in Pichia pastoris by utilizing the alcohol oxidase gene promoter. TNF expression level in certain transformants accounted for as much as 36% of the soluble protein. TNF expression was stably maintained during high cell density fermentation (100 g dry cell weight/liter) resulting in a TNF production level of 6–10g/liter. TNF contained in P. pastoris cell lysates was biologically active as determined by its cytotoxic effect on murine L-929 fibroblast cells and the bioactivity was retained for at least 6 months in the lysates stored frozen at −20 °C in the presence of protease inhibitor PMSF. TNF expressed in P. pastoris was recognized by monoclonal antibodies prepared against recombinant Escherichia coli derived TNF. TNF purified from P. pastoris has the expected N-terminal amino acid sequence and specific activity of 107 units/mg protein. TNF analogs were also expressed at levels comparable to that of native TNF. Three of the four analogs were insoluble when produced in P. pastoris.