Results are given of the examination for blood of 3214 milk samples from 935 cheese factory supplies. Over 75% of the milks contained less than 2·5 ppm. blood. Excessive blood was due to factors largely controllable by an efficient farmer, and so may be used as an indicator of poor dairy herd management and shed hygiene. No evidence could be obtained that the concentrations of blood likely to be found in commercial milk have any adverse effect on the keeping quality of whole milk or butter.