The platelet attachment and activation potential of a variety of treated, human root surfaces was studied employing an established in vitro method for determining thrombogenicity. In addition, scanning electron microscopic observations were made of the morphological appearance of blood components in initial clot formation on root surfaces containing periodontal ligament (PDL). For the platelet study, the crowns were removed from freshly extracted teeth with and without periodontal disease (PD) and the roots were sectioned. The following surface conditions were created: (1) PDL present, (2) PD, (3) PD planed, (4) PD planed and demineralized, (5) Condition 4 treated with collagenase. All conditions were incubated with Indium-1 ll-labeled platelets with and without the addition of prostacyclin, an inhibitor of platelet activation. It was observed that the greatest number of attached platelets were found in Condition 1, that platelet activation was significantly enhanced in Condition 4 and this effect was reversed by Condition 5. The scanning electron microscopic observations were made on freshly extracted teeth with an intact PDL in which the roots were sectioned and either reinserted immediately into the extraction site or incubated in platelet-rich plasma. It was seen that platelets were involved early in clotting and activated by this surface. This study suggests the possible use of platelet attachment and activation as an indicator for root surface thrombogenicity and perhaps of the fibrous attachment potential.