SummaryShort-term (0-30min) pulmonary and systemic vascular responses of oophorectomized ewes infused intravenously with a trichloroacetic acid (TCA) extract of a type III (strain 878) group B streptococcus (GBS) were studied. TCA-878 extract induced significant pulmonary hypertension, reduction in femoral artery pressure and reduced femoral artery PO2. These responses were similar to those observed after the infusion of sublethal doses ofE. coliendotoxin and could be prevented by priming the animal with ibuprofen or indomethacin. Ewes rechallenged 48 h after their initial dose of TCA-878 extract experienced pulmonary arterial pressure significantly higher than those induced by the first infusion. Similar augmented responses were not seen after rechallenge with endotoxin. Larger doses of TCA-878 extract resulted in increasingly higher pulmonary arterial pressure. We conclude that the venous infusion of TCA extracts of GBS-878 induces significant pulmonary hypertension in sheep and that this response may be mediated by prostaglandins. The vasoactive substance extracted from these streptococci could play a role in promoting the vascular instability experienced by the human neonate with early onset GBS disease