Bovine mastitis still causes great loss despite modern treatment, and discussion of this subject is therefore particularly appropriate to the Society of Animal Production. Data obtained before modern treatment became available, in several countries where dairying is intensive, showed that some 3-4 per cent, of dairy animals were culled annually on account of mastitis, that at any given time about 10 per cent, had some obvious abnormality of milk or udder, and that a high percentage of animals—20-40 per cent, in many areas and countries—were infected with one or other of the recognised mastitis bacteria, the majority with Streptococcus agalactiae. Much of this has been overcome by modern treatment, and in some areas Str. agalactiae mastitis has already been eradicated from large numbers of herds. There is no doubt, however, that in many other areas mastitis remains a cause of heavy loss of yield—a loss which could largely be removed by more active measures for its control.