Summary. A monoclonal antibody (anti‐Fn2) was prepared which was reactive with both plasma fibronectin and fibronectin located within the platelet alpha granule. Immunoblotting analysis, on thermolysin digestion fragments of fibronectin, identified two immunoreactive fragments of M_r 145 kDa and 155 kDa which are known to contain a cell and DNA binding region. Anti‐Fn2 was found to inhibit binding of fibronectin to platelets and DNA. Functional platelet studies, measuring platelet aggregation and ¹⁴C‐serotonin release in washed platelet systems, demonstrated the ability of anti‐Fn2 to totally inhibit low dose thrombin and low‐dose collagen induced platelet aggregation and serotonin release. Anti‐Fn2 partially inhibited platelet aggregation induced by ADP (10 μM) and arachidonic acid, but had no effect on platelet aggregation induced by high‐dose thrombin or by the calcium ionophore A23187.

These studies indicate that fibronectin participates in platelet aggregation and release induced by a range of agonists and suggest that it has a more important involvement in platelet function than previously described.