Platelet adhesion to low-density fibrinogen induces greater protein tyrosine phosphorylation than adhesion to high-density fibrinogen. Platelets were allowed to adhere to fibrinogen (fbg)–coated or collagen (coll)–coated wells for 1 hour. After washing, adherent platelets were lysed in a buffer containing phosphatase inhibitors as described in “Materials and methods.” (A) Equal amounts of protein were subjected to electrophoresis and immunoblotting with mAbs specific for phosphotyrosine. Phosphotyrosine staining of proteins was less intense in platelets adherent to high-density compared with low-density fibrinogen; a protein of Mr approximately 100 kDa demonstrated approximately 50% less intense staining in platelets adherent to high-density than to low-density fibrinogen. For comparison, phosphotyrosine staining of proteins from platelets in suspension prior to adhesion is shown. (B) Equal amounts of protein lysates were used to immunoprecipitate FAK. Immunoprecipitated proteins were analyzed by immunoblotting for phosphotyrosine. Thereafter, the membranes were stripped and reanalyzed with antibody to FAK, to verify that the amounts of immunoprecipitated proteins were equal in all lanes. Results shown are representative of 3 independent experiments.