Post-translational limitations in the endoplasmic reticulum during recombinant monoclonal antibody production are an important factor in lowering the capacity for synthesis and secretion of correctly folded proteins. Mammalian protein disulfide isomerase (PDI) has previously been shown to have a role in the formation of disulfide bonds in immunoglobulins. Several attempts have been made to improve the rate of recombinant protein production by overexpressing PDI but the results from these studies have been inconclusive. Here we examine the effect of (a) transiently silencing PDI mRNA and (b) increasing the intracellular levels of members of the PDI family (PDI, ERp72, and PDIp) on the mRNA levels, assembly and secretion of an IgG4 isotype. Although transiently silencing PDI in NSO/2N2 cells suggests that PDI is involved in disulfide bond formation of this subclass of antibody, our results show that PDI does not control the overall IgG4 productivity. Furthermore, overexpression of members of the PDI family in a Chinese hamster ovary (CHO) cell line does not improve productivity and hence we conclude that the catalysis of disulfide bond formation is not rate limiting for IgG4 production. Biotechnol. Bioeng. 2010;105: 770-779. (C) 2009 Wiley Periodicals, Inc.