Differential processes of immunoglobulin (Ig) gene-transcribing cells were investigated in the thymus of BALB/c male mice from neonatal to 40 weeks old. Both membrane-bound type IgM (IgM_m) and total IgM (IgM_t) transcriptions were detected by realtime PCR and the IgM_m/IgM_t ratios were determined. The ratio was approximately 1 to 1 week after the birth, and then decreased gradually following to age increase. Kinetics was applied in order to analyze the result with supposing that the transcription quantity was proportional to the cell number, that immature cells transcribing membrane-bound type IgM_m continued to be supplied from stem cells in vivo, and that the concentration of total cells transcribing IgM_t was unchanged in the thymus with age. Immature IgM_m cells were thought to differentiate either to ones transcribing secretory type IgM (IgM_s) or to other class 1g-transcribing by recombining heavy-chain genes. Theoretical values were in fair agreement with experimental values of log IgM_m/IgM_t on the first order concurrent reaction process and the rate constants for each process were determined
