Contact between normal human B-cell precursors and stromal cells increases tyrosine phosphorylation of multiple stromal cell but not B-cell precursor proteins. Stromal cells and normal human CD10⁺ CD19⁺ surface IgM⁻ B-cell precursors were incubated at 37°C, collected, lysed, electrophoresed, and immunoblotted as detailed in the Materials and Methods. (A) Stromal cells in medium for 5 minutes (−) or in contact with B-cell precursors for the times indicated (in minutes). (B) B-cell precursors in medium alone (−) or in contact with stromal cells for the times indicated (in minutes). (Upper panel) Antiphosphotyrosine immunostaining. (Middle panel) Anti-vav immunostaining. (Lower panel) Anti–β-tubulin immunostaining. Asterisks indicate the major stromal cell protein species demonstrating consistent increases in tyrosine phosphorylation. Results are representative of two individual experiments.