Integrin β3 is tyrosine-phosphorylated after the costimulation with soluble and solid-phase fibrinogen. D3-detached cells (0), the cells on fibrinogen plate incubated for 30 minutes (30), and the cells on fibrinogen plate after the additional stimulation with 1 mg/mL soluble fibrinogen for 5 minutes (30+Soluble Fg.) were lysed in 0.05% SDS-containing buffer and immunoprecipitated with anti-integrin β3 MoAb. Aliquots of each condition were equally divided and electrophoresed on the same gel in duplicate. Immunoblotting analyses were performed as described in the Materials and Methods. Results are representative of 3 experiments.