We studied the effects of an infusion of Serratia marcescens endotoxin on hemodynamic function, white blood cells, platelets, and the plasma levels of thromboxane B2 (TXB2) and 6-ketoprostaglandin F1 alpha (6-keto-PGF1 alpha) in awake monitored sheep. The animals were immunized using the core glycolipid (CGL) fraction of Escherichia coli J5 0111. An additional group of animals was studied after passive transfer of immune serum from the actively immunized CGL animals. Active immunization with CGL was effective in preventing the hematologic and hemodynamic changes as well as the elevations in TXB2 and 6-keto-PGF1 alpha seen during gram-negative endotoxemia. Passive immunization also reduced some of the deleterious effects of endotoxin infusion but was less effective than active immunization. This study demonstrates the potential of cross-specific immunization to protect against the cardiopulmonary alterations caused by gram-negative endotoxemia.