Pulmonary vascular responsiveness and lung homogenate activity of alloimmune hemolytic mice were similar to sickle mice and not to hemizygous sickle controls (mean ± SEM for n = 4 to 6 per group). A mouse model of acute hemolytic transfusion reaction was studied during alloimmune hemolysis by microcardiac catheterization. Because mice studied on the third day of hemolysis did not appear to have greater abnormality in the parameters measured than mice studied on the first or second days of hemolysis, data were pooled together. (A) Vasodilation to inhaled nitric oxide at 4 ppm was significantly blunted in alloimmune hemolytic mice compared with hemizygous controls (P < .05). (B) Vasodilation to bradykinin infusion was significantly blunted at 3 μg/kg in alloimmune hemolytic mice (P < .05). (C) The endothelium-independent agent CGRP (0.3 nmol/kg intravenous bolus) produced the same vasodilation response in alloimmune hemolytic mice and sickle mice as in hemizygous controls (P = NS). (D) eNOS assay of lung homogenate was significantly lower in alloimmune hemolytic mice than in hemizygous controls (P < .05), similar to sickle mice. (E) Arginase assay of plasma and lung homogenate was significantly elevated in both alloimmune hemolytic mice and sickle mice compared with hemizygous controls (P < .05). (F) Luminol assay of lung homogenate indicated significantly elevated reactive oxygen species in alloimmune hemolytic mice and sickle mice compared with hemizygous controls (P < .05). Statistically significant results are indicated by an asterisk (P < .05 versus hemizygous controls).