BiP, GRP94 and PDI, three endoplasmic reticulum (ER) based proteins are involved in the maturation of secretory proteins and might represent a bottleneck in the secretory pathway of monoclonal antibodies (MAB). With the three hybridoma cell lines tested, MAB production kinetics were significantly increased for the batch cultures done in serum-free medium (SFM) with respect to those done in serum-containing medium (SCM). It could be established that there was a correlation between the cellular levels of PDI and GRP94 and the specific MAB production rate. With respect to BiP, no correlation with the MAB production rate was observed. The non-producing myeloma cell line X63, used as a reference, showed increased cellular PDI levels when cultivated in SFM. However, in this cell, the cellular GRP94 levels were not significantly influenced by the medium composition. It was concluded that SFM induced an increase of cellular PDI levels and this elevation seemed to be responsible for the increase in the specific MAB production rates. On the other hand, only MAB producing cells showed an increase in the cellular GRP94 levels which might be a result of increased MAB synthesis. Indeed, 1.13.17 cultivated in SFM supplemented with serum showed a significantly reduced (about 50%) specific MAB production rate in comparison to 1.13.17 cultivated in non-serum supplemented SFM. The cellular PDI and BiP levels did not vary between these conditions of culture, whereas the cellular GRP94 level was about two-fold lower in 1.13.17 cultivated in SFM when supplemented with serum than in 1.13.17 cultivated in SFM without further supplementation. These results are discussed with respect to the medium composition as well as in the context of apparent and potential bottlenecks within the secretory pathway of MAB.