Immunofluorescent Staining: Chicken peripheral blood mononuclear cells were first incubated with purified Mouse anti-Chicken Bu-1a and then counter-stained with Goat anti-Mouse IgG-FITC. Small lymphocytes were then gated using light scatter criterias and analyzed on a FACScan (TM) flow cytometer (BDIS, San Jose, CA). Amount Used: 1 ug/10e6 cells.