The correlation between haptoglobin (Hp) and mucoprotein or sialic acid in sera from diseased cattle was examined to evaluate the clinical application of serum Hp. First, the ranges of mucoprotein, sialic acid and hemoglobin binding capacity (HbBC) of 46 healthy cattle were determined. Then, levels of these reactants in 35 cattle suffering from various diseases were measured. The frequency of the abnormal values was 94.3% in mucoprotein, 45.7% in sialic acid and 82.9% in HbBC in diseased cattle. There was a significant correlation between mucoprotein and sialic acid, and between HbBC and mucoprotein of sialic acid. Some correlation between mucoprotein and neutrophils or α-globulin was also observed. In spite of high levels of mucorprotein and sialic acid, lower HbBC than expected levels were often observed in inflammatory disorders. Such cases might be complicated with some hemolytic disorders such as piroplasmosis. Determining serum HbBC together with other acute phase reactants might be useful for evaluating inflammatory diseases complicated with hemolytic disorders.