Time course of changes in p72^syk activity induced by NNKY5-5. Platelets were activated with 10 μg/mL NNKY5-5 and the reaction was terminated by adding lysis buffer after the indicated periods. Platelet proteins associated with p72^syk were immunoprecipitated with an anti-p72^syk antibody. The sample was then split into portions for an in vitro tyrosine kinase assay and for Western blotting with an anti-phosphotyrosine antibody to detect the level of autophosphorylation of p72^syk. (A) Western blot. (B) In vitro kinase assay. The arrowheads represent the bands presumably derived from the heavy chain of the anti-p72^syk antibody.