A gene coding for human parathyroid hormone (hPTH) was synthesized and cloned into a yeast expression and secretion vector containing the mating factor pre-pro leader sequence and the galactose-inducible promoter, GAL10. The intact hPTH(1-84) was found to be secreted into the culture medium. As observed in the previous reports on the secretory production of hPTH in yeast, however, the proteolytic cleavage occurred as the culture proceeded, resulting in a significant loss of the intact hPTH. Attempts were therefore made to reduce the extent of proteolysis by simply controlling the culture conditions. The proteolytic cleavage was significantly reduced by the addition of an excess amount of L-arginine (≥0.2M) to the culture medium, which resulted in a marked improvement in the yield of intact hPTH. To examine whether L-arginine affects the activities of intracellular proteases such as KEX2 endoproteinase or extracellular proteases, the proteolysis experiments were performed by incubating the commercial intact hPTH in a yeast host culture supernatant. The results demonstrated that L-arginine at high concentrations reduced the rate of hPTH proteolysis by inhibiting extracellular proteases.