Precursor B (pncB) cells in bone marrow of normal and B cell-deficient mutant mice were analyxed for the expresslon of V&&5 surrogate light chain (SL). The surface expression of SL Is confined to the early stages (pro-B and pre-BI) of pre-B cell development and becomes undetectable once p heavy chain (@ I) is produced. The cell-cycle analysis revealed that cytoplasmlc pH+ large cells (large pre-MI),-30% of which coexpressed SL In the cytoplasm, were most actively cycling, whereas cytoplasmlc pH+ small cells (small pre-B-II) were SL-and not In cycle. The analysts of pre-B cells in B cell-deficient mice suggests that the large pre-B-II stage is a critical step for the selection and ampllflcation of cells carrying functionally rearranged pH genes.