In this thesis the influence of the exogenous antigenic environment on the murine B cell repertoire has been studied. This was done by using neonatal and adult germfree BALBjc mice fed a chemically defined ultrafiltered 'antigen-free' diet (GF-CD mice) and conventional syngeneic control mice fed natural ingredient diet (CV-NI mice). The actual B cell repertoire was studied by enumerating the total numbers of lgsecreting cells and the frequencies of antigen-specific lg-secreting cells and by analysis of the serum lg levels.