Changes in the patterns of glycosylation of canine haptoglobin have recently been demonstrated by isoelectric focusing and immunoblotting. Fucosylated fractions of haptoglobin were identified by selective binding of a fucose-specific lectin. In this study, similar changes were found in the serum of 86 of 137 dogs with various inflammatory, autoimmune and neoplastic diseases. Major changes, including fucosylation, were observed in 40 of the dogs and were most frequent in association with autoimmune haemolytic anaemia. All 40 cases had markedly increased concentrations of haptoglobin and decreased concentrations of haemoglobin. Minor changes were found in the other 46 dogs, whereas no changes in glycosylation were detected in the serum of 40 healthy dogs.