SummaryNormal hemostasis depends in part on the balance achieved between proaggregatory and prothrombotic platelet thromboxane A2, measured as its stable end-product thromboxane B2(TXB2), and vascular prostacyclin (PGI2), which inhibits platelet aggregation and is antithrombotic. Cystathionine-β-synthase deficiency is characterized by a high frequency of thromboembolic disease. We therefore studied,in vitro, the effects of homocysteine and related compounds on platelet TXB2and vascular PGI2formation.In paired samples of platelet rich plasma, which had been preincubated with L-homocystine (1 mM), mean production of the two platelet cyclooxygenase products, TXB2and 12-hydroxy-5,8,10-heptadecatrienoic acid increased significantly from control levels [13.6% ± 1.9 to 19.8% ± 2.1 (P< 0.02) TXB2and 29.8% ± 4.2 to 39.4% ± 4.1 (P< 0.01) HHT]. In the presence of D.L-homocysteine (1 mM), mean platelet TXB2and 12-hydroxy-5,8,10-heptadecatrienoic acid production was also significantly increased [12.7% ± 1.5 to 16.9% ± 1.5 (P< 0.01) TXB2and 27% ± 4 to 31% ± 4.1 (P< 0.02) HHT]. Cystine, cysteine, or methionine (1 mM) did not have similar effects in this test system. Homocysteine and homocystine were without effect on the synthesis of vascular PGI2by umbilical artery segments [control, 0.22 ± 0.03 to 0.21 ± 0.03 ng/mg with D.L-homocysteine and 0.20 ± 0.04 control to 0.19 ± 0.04 ng/mg with D.L-homocystine]. A homocyst(e)ine-induced increase in platelet thromboxane production in the absence of an increase in vascular prostacyclin, if presentin vivo, may contribute to the vascular thromboses characteristic of human homocystinemias (homocystinurias).