Transduction ability of the proB-cell surface complex as compared with that of the preB- and B-cell receptor. The proB (JEA2), preB (LAZ 221), and B (Namalwa) cells were loaded with Indo-1 as described in Materials and Methods. Anti-VpreB 4G7 (30 μg/mL), anti-CD19 (30 μg/mL), anti-μ (30 μg/mL), or irrelevant IgG1 (30 μg/mL) MoAbs were added and cross-linked by a goat antimouse IgG+IgM (H+L) at the times indicated by arrows. Fluorescence variation of indo-1 was measured by FACstar and the percentage of activated cells was determined by the MultiTime software (Phoenix Flow Systems, San Diego, CA).