Three-color immunofluorescence analysis of normal bone marrow cells shows cell surface expression of ΨL on both the proB and preB compartments. Fetal bone marrow lymphoid cells were incubated with PerCP-labeled anti-CD34, FITC-labeled anti-CD19, and PE-labeled anti-VpreB 4G7 or PE-labeled irrelevant γ1 control MoAbs. Both the CD34⁺VpreB⁺ (R1) and the CD34⁻VpreB⁺ (R2) cells were CD19⁺, identifying proB- and preB-cell compartments, respectively.