The presence of serum haemoglobin reactive protein (HRP) as an indicator of infection has been suggested. The serum HRP levels in varying groups of cattle were measured. A reference range, established on 233 cattle, of 0–10ώmol/1 (as methaemoglobin binding capacity) was used. Forty eight of 104 cows that had recently aborted had raised serum HRP levels. Of 37 cows that had recently aborted, 26 had a leucocyte picture typical of bacterial infection and 18 of these had raised serum HRP levels. Eleven cows with a normal leucocyte picture had normal serum HRP levels. Seventeen cattle, with clinical infection and leucocyte pictures typical of bacterial infection, all had raised HRP levels. Of 133 cattle in which infection was considered a possibility by the attending veterinarian, 53 had an abnormal leucocyte picture with a raised serum HRP level. The mechanisms of a raised serum HRP level are unknown but, at the clinical level, its association with pyogenic infection make it a useful test in the confirmation of pyogenic infection.