Fed-batch fermentations were performed to overproduce human parathyroid hormone (hPTH) in a Saccharomyces cerevisiae mutant, deficient in yeast aspartic protease 3 (YAP3), developed to minimize the proteolytic cleavage of hPTH. The secretion of hPTH, which was expressed under the control of GAL10 promoter, was directed by the mating factor alpha pre-pro leader sequence. The intermittent feeding of medium containing carbon mixtures of galactose and ethanol was the feeding strategy, which resulted in the maximum production of intact hPTH of similar to 188 mg/l. Although a yap3-disrupted mutant was used as a host, most of the secreted intact hPTH was degraded at later stages of fermentation. To minimize the loss of intact hPTH, it is important to terminate the fermentation when the concentration of intact hPTH reaches the maximum. (C) 2000 Elsevier Science Ltd. All rights reserved.