Haptoglobin (Hp) was isolated from equine serum by ammonium sulphate precipitation, anion-exchange chromatography and gel filtration. Equine Hp which migrated to the α₂-globulin region in electrophoresis, contained 2 fractions with molecular weights (NW) of 108, 000 and 105, 000, and each fraction consisting of 2 subunits. Quantitative measurement of Hp in equine serum was performed by the single radial immunodiffusion method using anti-equine Hp serum. In clinically normal horses, the highest concentration of serum Hp was found in newborn foals and a high value was maintained until 12 months of age. The concentration then decreased with age. Normal Hp values were 5.25±2.36 mg/ml in foals (≤12 months old), 2.19±1.54 mg/ml in adult horses (≥18 months old) and 3.62±0.81 mg/ml in all horses. Serum Hp concentration in mares during the perinatal period in comparison with the normal adult female was high for 4 months pre-partum, a passing increase at delivery, and then decreased at 2 weeks post-partum returning to normal within 1 month of delivery. In horses with experimentally-induced inflammation, serum Hp concentration began to increase immediately after treatment and reached the highest value, 1.5 to 9 times higher than those of pre-treatment at 2 to 5 days, then decreased within 4 weeks. It was also elevated in most cases of horses with clinically inflammatory signs.