The serum concentration of haptoglobin was measured in 51 cows with toxic puerperal metritis which were being treated with one of three different antimicrobial regimens. The mean concentration of haptoglobin was 19.0 mg/dl on the day that the treatments began and declined steadily during the five day treatment period to a mean concentration of 7.35 mg/dl. There was no correlation between the serum haptoglobin concentrations and the rectal temperatures of the cows during the five days.