Immunofluorescent Staining: Cells from BALB/c fetal liver were c ultured with IL-7-producing stromal cells for 3 days at 37°C, stained with Rat anti-Mouse BP1/6C3-PE, and analyzed on a FACScan (TM) flow cytometer (BDIS, San Jose, CA). Amount Used: < / = 0.2 ug/10e6 cells.