Abstract Both canine plasma and canine platelets have been reported to be unreactive in the ristocetin-induced platelet aggregation reaction that is commonly used to study the platelet-von Willebrand factor interaction. Because canine plasma was recently found to have ristocetin cofactor (RCF) activity with human platelets, we studied the RCF reactivity of canine platelets. Although canine PRP did not aggregate with ristocetin, ristocetin did induce aggregation of canine gel-filtered platelets (GFP), apparently because inhibitory plasma proteins, especially albumin, had been separated from the platelets during gel filtration. When human and canine GFP were washed, they lost their responsiveness to ristocetin. Although the ristocetin responsiveness of washed human GFP was completely restored by both human and canine factor VIII, washed canine GFP aggregated with ristocetin only in the presence of canine factor VIII. Human platelets, when fixed in paraformaldehyde aggregated well with ristocetin and either human or canine factor VIII. In contrast, paraformaldehyde-fixed canine platelets aggregated very poorly with ristocetin and factor VIII. These studies show that the ristocetin-RCF-platelet interaction functions well in the dog, although the reaction is much more species specific than the similar human reaction.