Severe combined immunodeficiency (SCID) mice were immunologically reconstituted by the transfer of fetal liver cells (FLC) from BALB/c (SCID-isoFLC mice) or C57BL/6 mice (SCID-alloFLC mice). The developmental process of the immune system in the peripheral blood (PB) was almost comparable between SCID-isoFLC and SCID-isoFLC mice. Analysis of the lymphoid organs and the PB from SCID-alloFLC mice indicated that slg⁺ B cells appeared and were distributed to the periphery within 2 weeks after the transfer of FLC. It was also suggested that precursor T cells entered the thymus before 4 weeks after the transfer, and were differentiated into mature CD4⁺ or CD8⁺ T cells which then migrated to the periphery by 6 weeks. All of mature lymphocytes in the periphery of the SCID-alloFLC mice were shown to express donortype H-2 antigens. Additionally, in the SCID-isoFLC mice, cell-mediated immunity such as rejection against alloantigens was functioning from 6 weeks, and humoral immune function was suggested by the detection of cells generating antigen-specific antibodies. We discuss that development of the immune system in SCID mice receiving transferred FLC was comparable to that normally seen in the fetal and neonatal stages.