During long-term continuous culture of the hybridoma cell line 11317, a better-producing subclone (11317-SF11), giving improved productivity, has been selected. The comparison of the original cell line (11317-DC) with this subclone revealed that although the growth patterns of both clones were similar, both in continuous and in batch cultures, considerable differences could be seen between the clones with respect to monoclonal antibody (MAB) accumulation, MAB production rate, the levels of mRNA coding for heavy and light chains of IgG, and some metabolic activities. In continuous culture as well as in batch culture, 11317-SF11 showed increased levels of mRNA coding for kappa and gamma chains compared with 11317-DC and/or a modified ratio of the mRNA species when compared to that in 11317-DC. Using pulse experiments, it could be established that the biosynthesis of both chains was augmented in 11317-SF11. Although the kappa and gamma mRNA levels were modified or inversed for 11317-SF11, the cells always synthetized more kappa than gamma chains. The overall increase in the synthetic activity of 11317-SF11 is suggested as one reason for the considerable increase of IgG productivity and product accumulation in continuous culture as well as in repeated batch cultures. Tests concerni ng meta belie activity revealed that 11317-SF11 had a predominantly glycolytic metabolism independent of growth requirements, whereas for 11317-DC the metabolism became increasingly glycolytic with increased growth. The antibody yield coefficient of 11317-SF11 on glutamine was significantly higher than that of 11317-DC for the continuous culture, whereas the antibody coefficients on glucose were almost similar for both clones under the different culture conditions used. Both antibody coefficients were considerablly influenced by the specific growth rate. All these facts together lead to the conclusion that subclone 11317-SF11 uses more of the energy available, or it was the energy and/or precursors available for the synthesis and production of MAB more efficiently than the original cell line. Although the levels of mRNA coding for heavy and light chains of IgG were modified, it could be confirmed that the overall regulation of MAB-synthesis and -production occurs post-translationally and that at higher growth rates, more biosynthetic activity is diverted to biomass production.