Previous work has shown that a human-antibody-producing recombinant CHO cell line did not increase its intracellular content of protein disulfide isomerase (PDI) and heavy chain binding protein (BIP) according to the increasing expression of antibody. It was also found that the intracellular assembly of light and heavy chain is a major limiting factor for overall cell specific productivity, as secretion rates improve with higher light chain expression levels and heavy chain accumulates intracellularly when too little light chain is present. As these CHO cells had a significantly lower intracellular PDI content compared to that of hybridoma cells, these results have led us to try to overcome the limitation in the posttranslational assembly in the endoplasmatic reticulum. Recombinant CHO cells were transfected with PDI or BIP alone or in combination, and the effect on intracellular light and heavy chain content and specific production rate was determined. Overexpression of BIP, both alone and in combination with PDI, reduced the specific secretion rate, whereas PDI, when overexpressed alone, caused an increase of product secretion rate.