Cancer sera have high levels of an abnormal form of haptoglobin (Hp) that can be extracted from blood using the fucose-specific lectin, Lotus tetragonolobus. In order to investigate the carbohydrate abnormality that is responsible for this effect, the monosaccharide composition of Hp has been measured in Hp preparations isolated from blood samples of healthy women and women with ovarian cancer. The fucose content (g/100 g protein) of Hp was considerably elevated (7-fold) in ovarian cancer; whereas the concentrations of the other sugars were not significantly increased. There were significant increases in the galactose and N-acetylglucosamine contents of the 'cancer' Hp if the levels were expressed relative to the mannose content. This latter finding suggests that there are more oligosaccharide branches on the 'cancer' Hp. The results indicate that Hp is extracted by lotus lectin from ovarian cancer sera because of the increased fucose levels and provide further support for a disturbance in fucose metabolism in cancer.