In humans (A), baboons (B), and monkeys (C), platelet aggregation with ristocetin at 1mg/ml was dose-and dilution-dependent. Ristocetin (at 1mg/ml) induced significantly greater aggregation in undiluted blood than in 75% diluted blood in all three species. Ristocetin induced significantly greater aggregation in 50% diluted blood than in 75% diluted blood only in humans. In all three species, ristocetin at 0.5mg/ml induced dilution-dependent aggregation, but significant differences were only observed in monkeys. Values of mean percentage (%) of aggregation are indicated in the figure. (*p<0.05, **p<0.01, ***p<0.001).