The Pichia pastoris yeast is used for recombinant secretory proteins production. The overexpression of assistant genes, the coding proteins involved in secretion, is one of the approaches used to improve the production of target proteins. PpPDI gene encodes P. pastoris yeast protein disulfide isomerase (Pdi). The aim of our study is to evaluate the effect of Pdi overproduction on the production of recombinant interferons. We obtain the yeast Pichia pastoris strains expressing PpPDI gene under the control of alcohol oxidase-1 (AOX1) promoter. The effect of the overexpression of the PpPDI gene on the production of heterogious proteins—interferon-alfa16 and chicken interferon-gamma—and on the cell growth rate is shown.