The in vitro effects of ZK 36 374, a new chemically stable prostacyclin (PGI2) analog, on platelet aggregation were studied and compared to those of PGI2. Significantly lower concentrations of ZK 36 374, versus prostacyclin, were required to inhibit collagen and epinephrine-induced aggregation in human platelet-rich plasma. In contrast, in rat and rabbit platelet rich plasma, PGI2 was more effective than ZK 36 374 in inhibiting the aggregation elicited by ADP and collagen. It is concluded that ZK 36 374 is a potent antiaggregatory compound and may be useful in the prevention of cardiovascular disorders.