Abstract The acquired immunopathology of lymphoid and parenchymal organs of University of California, Davis (UCD), line 140 White Leghorn chickens, previously demonstrated to have an inherited dysgammaglobulinemia with associated autoimmune phenomena, is described and compared to normal UCD chicken lines 159, 011, and 440. Although, at autopsy, morbid birds had signs of anemia, congestive heart failure, and splenomegaly, the most prominent features were immunologic in origin. Specifically, there was evidence for immune complex disease, including deposition of immunoglobulin in renal glomeruli, vasculitis, and synovitis. However, of particular importance were age-dependent abnormalities of thymus and bursa of 140 but not those of control chickens. Thymi of 1-year-old birds were dramatically enlarged with microscopic appearance of both hyperplasia and thymoma; the thymi of birds less than 6 months of age were normal. In contrast, bursa from neonatal birds of line 140 but not those of control chickens were markedly underdeveloped and revealed cystic degeneration of bursal epithelium. Furthermore, the size of the bursa and the mitotic index of bursacytes of line 140 were significantly reduced compared to controls. Similarly, although the primary response to SRBC and Brucella antigen was of comparable magnitude in young line 140 and control birds, the percentage of mercaptoethanol-resistant secondary response was lower in line 140 animals. This anomaly of line 140 bursa is unique and suggests that this syndrome of inherited acquired dysgammaglobulinemia of birds, and perhaps acquired agammaglobulinemia of humans, is due to a primary maturational defect of bursa.