The stability of KR12H-1 transfectoma in regard to chimeric antibody production was examined during long-term, repeated batch culture without selection pressure using antibiotics. Both serum-supplemented and serum-free media were used. Regardless of the medium used, the specific antibody productivity ($q_{Ab}$) of transfectoma decreased by 60-88% during 70-day culture. This loss of antibody productivity was not due mainly to the appearance of a nonproducing population (NP) of transfectoma. The percentage of a producing population (P), which was monitored by the limiting dilution method, remained over 90% until the end of culture, indicating that the $q_{Ab}$ of P decreased during the culture. Flow cytometric data also showed the increase of cell population with low fluorescence intensity during culture, indicating that the intracellular antibody content of P decreased. The subclones of P obtained at the end of long-term culture were further characterized. Compared with the $q_{Ab}$ of P at the beginning of long-term culture, the $q_{Ab}$ of most P subclones was significantly low, confirming that the loss of antibody productivity was due mainly to the decreased $q_{Ab}$ of P during long-term culture. The decreased antibody gene copy number of P subclones was found to be partly responsible for the decreased $q_{Ab}$ of P during the long-term culture.
