We report a new procedure to express recombinant human activin A using the methanolic yeast, Pichia pastoris. Optimization of culture procedures has involved comprehensive examination of the effects of culture vessel shape, volume of broth in the induction and expression cultures, methanol concentration, culturing temperature, and pH of the expression cultures. After this optimization, as well as modification of the native cleavage sites, a laboratory scale procedure has been established which routinely produced 2-10 mg/L amounts of this vital growth factor in the highly efficient, eukaryotic yeast system. This system avoids the need to produce this protein and similar TGF-beta proteins in mammalian cell lines which, in addition to being costly, produce many native binding partners of these cystine knot proteins, a factor which can dramatically affect yields of the target protein. (C) 2010 American Institute of Chemical Engineers Biotechnol. Prog., 26: 372-383, 2010