To design the scheme of large-scale production of chimeric antibody for the postexposure prophylaxis of hepatitis B virus (HBV) infection, the stability of transfectomas (H69K-1 and 6-31) in regard to antibody production was examined during a long-term, repeated fed-batch culture without selection pressure using antibiotics. Although the H69K-1 transfectoma was more stable than the 6-31 transfectoma, both displayed gradual decreases in specific antibody productivity (q(Ab)) for the first several weeks of cultivation. During this period, q(Ab) was decreased by 40% to 50%. This loss of q(Ab) was due mainly to the appearance of a nonproducing population (NP) of transfectoma, which was monitored throughout the culture by flow cytometry and the limiting dilution method. However, an NP did not overtake the culture and was balanced with a producing population (P) of transfectoma, resulting in stable antibody production. The subclones of NP obtained at the end of long-term culture were further characterized by reverse transcription-polymerase chain reaction assay of the heavy and light chain mRNA. All the subclones of NP derived from H69K-1 transfectoma had only light chain mRNA. On the other hand, an NP in the 6-31 transfectoma culture was heterogeneous. Some subclones of NP derived from 6-31 transfectoma had only heavy chain mRNA and other subclones had only light chain mRNA. Taken together, the results obtained here suggest that selection pressure is necessary for a long-term, continuous culture, because stable antibody production in a long-term culture was achieved only after a significant loss of antibody productivity. Accordingly, a batch culture appears to be more appropriate for large-scale chimeric antibody production without selection pressure.