Contact with RAMOS B cells increases tyrosine phosphorylation of multiple bone marrow stromal cell proteins. Stromal cells were incubated in medium alone (−) or in contact with RAMOS-wt (wt) or RAMOS-4c (4c) cells for 5 minutes at 37°C. Cells not in contact were aspirated and discarded. Stromal cells and RAMOS cells were then collected separately, lysed, and electrophoresed as detailed in the Materials and Methods. Twenty-five micrograms of stromal cell or RAMOS protein were electrophoresed in each lane. (Upper panel) Antiphosphotyrosine immunoblotting. (Middle panel) Anti-vav immunoblotting. (Lower panel) Anti-ERK2 immunoblotting. Asterisks indicate the major stromal cell proteins demonstrating consistent increases in tyrosine phosphorylation in greater than 20 individual experiments.