Human hemoglobin cross-linked between the alpha chains with bis (3,5-dibromosalicyl) fumarate (DBBF-Hb) was exchange transfused in swine and the histomorphologic changes were evaluated. Following exchange, animals were euthanized and tissues were taken for light and electron microscopy at 7.5 hours and days 1, 4, 7, and 15. Consistent hepatocellular and renal epithelial cell changes were seen. Hepatic injury, evident at 7.5 hours as cellular vacuolization, progressed to necrosis and acute inflammatory cell infiltration by days 1 and 4, was resolving by 7 days and was completely resolved by day 15. Cytochemical stains for iron and hemoglobin revealed positive material in Kupffer cells, endothelial cells, and necrotic hepatocytes. Rabbit anti-human hemoglobin antibody staining revealed immunoreactive material diffusely present at days 1 and 4 and limited to solitary hepatocytes by day 15. Kidney injury began as proximal tubular epithelial vacuolization and intraluminal casts progressing to tubular necrosis by 24 hours, with resolution by day 15. Iron and hemoglobin stains demonstrated these materials in the early lesions. Immunocytochemistries demonstrated human hemoglobin that remained as late as day 15. Electron microscopy revealed degeneration and regeneration of epithelial cells. The renal lesions were consistent with hemoglobinuria. The liver lesion was less well defined but was self limited.