Background: The infusion of pig growth factor-mobilized peripheral blood leukocytes (containing 1 to 2% progenitor cells) (pPBPC) into baboons is associated with a thrombotic microangiopathy, which results from a direct effect of these pig cells on platelet aggregation. Ajoene is a synthetic derivative of garlic that inhibits aggregation of human platelets induced by all known agents. To assess its potential use in models of xenotransplantation, this agent was tested for its effect on baboon platelet aggregation in vitro and in vivo. Methods: In vitro studies: Baboon platelet aggregation assays, using adenosine diphosphate (ADP) (20 or 40 μM) or collagen (12.5 μg/ml), were performed after incubation with ajoene (0 to 150 μg/ml) or dipyridamole (0 to 200 μg/ml). Platelets were also incubated with pPBPC (5 × 106 cells) without or with ajoene in the absence of a known agonist. In vivo studies: Baboons received either a single intravenous dose of ajoene (10 to 25 mg/kg) or dipyridamole (0.8 mg/kg), or repeated doses of both agents at 2 to 3 h intervals. Platelet-rich plasma was obtained for platelet aggregation assays at time points up to 4 h post-drug administration. Results: In vitro, platelet aggregation was inhibited by 95% (ADP assay) and 89% (collagen assay) by ajoene at concentrations of ≥75 μg/ml. Dipyridamole had no effect at concentrations of < 100 μg/ml, but inhibited aggregation almost completely at higher concentrations. Ajoene inhibited the aggregation caused by pPBPC by 33 to 50%. In vivo, platelet aggregation was completely inhibited for 2 h by ajoene at 25 mg/kg. Dipyridamole at 0.8 mg/kg reduced aggregation by 20% for 15 min, but the effect was lost by 60 min. In combination, the two agents prolonged inhibition marginally. Repeated doses of both agents at 2 h intervals maintained complete inhibition of aggregation, but did not do so when the interval between doses was extended to 2.5 or 3 h. Combined therapy was not associated with any bleeding complications. Conclusions: Although ajoene is a powerful inhibitor of platelet aggregation, the need for repeated administration and its partial effect on pPBPC-induced platelet aggregation would suggest that it may be of only limited value in preventing the thrombotic microangiopathy that develops when pPBPC are infused into baboons. However, it would seem worthy of further investigation when used in combination with other agents.