Bovine factor VIII aggregates human platelets either in a strong single wave at high concentration (10 mug/ml platelet suspension) or in two waves at low concentration (0.2-I mug/ml). The strong single wave of aggregation is not associated with release of [14C]serotonin or beta-glucuronidase; the high concentration does not induce retraction of reptilase-clotted platelet-rich plasma. Wtih the low concentration, relase of [14C]serotonin is observed just prior to the onset of the second wave of aggregation; release of beta-glucuronidase does not occur at any moment. The low concentration of bovine factor VIII induces moderate retraction of reptilase-clotted platelet-clotted platelet-rich plasma, which is inhibited by acetylsalicylic acid, indomethacin and apyrase, indicating that it is a consequence of release of platelet adenosine-5'-diphosphate. It has previously been suggested tht carbohydrate groups are involved in the human platelet-bovine factor VIII interaction, since galactose oxidase and periodate oxidation abolish the platelet aggregating activity of bovine factor VIII. The present study shows that these oxidizing substances also induce a degradation of bovine factor VIII, so that the exact role of carbohydrate groups in the aggregation process remains to be established.