The nb/nb mouse with a hereditary hemolytic anemia is an animal model of hemolysis-induced gallstone disease. These anemic mice have hepatomegaly and form calcium bilirubinate gallstones. We undertook this study to: (a) examine the histopathology of the liver and gallbladder in nb/nb mice and (b) assess the influence of hemolysis per se on liver and gallbladder histology by transplanting nb/nb bone marrow into another genotype W/Wv. Livers and gallbladders obtained from male nb/nb and control mice of similar age were stained with hematoxylin and eosin. Gallbladders were also stained with alcian blue (pH 2) and periodic acid-Schiff for acidic and neutral glycoproteins, respectively. Volume densities of the extralobular (blood vessels) and lobular (hepatocytic and sinusoidal) components of the liver and glands of the gallbladder neck were determined by standard morphometric techniques. The liver mass of nb/nb mice was 25% greater than that of control mice (1.79 ± 0.26 S.D. gm vs. 1.43 ± 0.23, p < 0.001). The actual hepatocytic mass of nb/nb and control mice was similar (1.35 ± 0.19 gm vs. 1.26 ± 0.21, p > 0.05). However, the sinusoidal compartment, representing extramedullary hematopoiesis, was three times greater in nb/nb than in control livers (0.40 ± 0.13 gm vs. 0.13 ± 0.05, p < 0.001). Overall, glandular concretions within the gallbladders were present at least twice as often in nb/nb than in control mice (p < 0.02). Bile pigment concretions occurred in only nb/nb gallbladders and were present three times more frequently in those with luminal gallstones (81%) than in those without stones (25%) (p < 0.02). The glandular density of nb/nb gallbladders was higher than that of control gallbladders (16 ± 21%, p < 0.02), and nb/nb gallbladders with luminal gallstones had a higher glandular density than nb/nb gallbladders without stones (88 ± 8% vs. 37 ± 20%, p < 0.05). Eight months after transplantation, recipient W/Wv mice had not developed gallstones but had marked hepatomegaly with a disproportionate increase in the sinusoidal compartment (27.4 ± 4.9%) compared with control W/Wv mice (9.90 ± 2.0%, p < 0.001). The glandular concretions and density in transplanted and control W/Wv gallbladders were similar. These data indicate that: (a) the hepatomegaly of nb/nb mice is due to extramedullary hematopoiesis while the morphological mass of hepatocytes is similar in control and nb/nb mice; (b) bile pigment concretions within nb/nb gallbladders precede luminal calcium bilirubinate stone formation; (c) glandular hyperplasia is a secondary event in hemolysis-induced gallstone disease, and (d) the hemolytic anemia is responsible for the hepatomegaly and gallstone formation, but the genotype determines the glandular density and concretions within the gallbladder prior to luminal gallstone formation.