p72^syk-associated tyrosine kinase activity induced by botrocetin-vWF. Platelets suspended in a buffer containing 1 mmol/L Ca²⁺ were activated with 3 μg/mL of botrocetin and 10 μg/mL of vWF for the indicated time intervals. Reactions were terminated with lysis buffer, and p72^syk was isolated by immunoprecipitation with anti-p72^syk MoAb. Immunoprecipitates were directly subjected to Western blotting using the antiphosphotyrosine MoAb, 4G10. The arrow represents the band presumably derived from IgG heavy chains. The data are representative of three experiments.