High concentrations of bovine factor VIII cause clumping of platelets into a few very large aggregates. This response is termed superaggregation. It is distinct from factor-VIII-induced agglutination but is also independent of both extracellular calcium ions and platelet energy metabolism. Neither agglutinating lectins nor aggregating agents, including thrombin, ADP, the ionophore A23187, and U46619, a prostaglandin analog, can induce superaggregation, even at very high concentrations. Washed platelets undergo superaggregation, and superaggregation does not increase the amounts of fibrinogen or albumin trapped by agglutinated platelets. It is not inhibited by membrane-stabilizing drugs or by colchicine or cytochalasin-B. Formaldehyde and glutaraldehyde prevent superaggregation without affecting the binding of radiolabeled factor VIII to the platelets. Superaggregated platelets are separated by approximately 50 nm and are not shape-changed or degranulated. In adenosine diphosphate (ADP) induced aggregation, the platelets are distorted and only 30 nm apart. Superaggregation is reversed by dextran sulfate, and the dispersed platelets are still able to respond to ADP. Our observations are consistent with the binding of high molecular weight multimers of bovine factor VIII to more than one receptor on each platelet, with superaggregation occurring through recruitment of additional receptors. This process may be interrupted by protein crosslinking reagents, such as formaldehyde and glutaraldehyde.