1. An account is given of the results obtained over periods of 14 and 12 months respectively from a scheme applied to two South African Friesland herds and aiming at the eradication of chronic streptococcus mastitis from these herds. 2. The scheme is based on periodic testing of all cows and heifers for mastitis by microscopic examination of smears prepared from incubated samples of milk, the application of infusion therapy to infected and suspected quarters, segregation in the stable between infected and non-infected animals, and the employment of ordinary hygienic precautions in the stable. 3. Details are given of the methods of diagnosis and treatment, of the control measures and the recording of results. 4. For the successful control of the disease the full active co-operation of the owner is essential. 5. Accurate evaluation of the results obtained from curative treatment is rendered impossible by such factors as the tendency of many infected quarters to discharge streptococci intermittently and not continuously, the unreliability of a single negative test, the fact that infection is not necessarily shown immediately after it is contracted, and the tendency of quarters which have been apparently cured to show a reappearance of streptococci in the milk. 6. As a result of infusion therapy systematically applied the infection among the cows in the two herds was reduced from 55·7 per cent. and 48·2 per cent. to 6·3 per cent. and 15·7 per cent. respectively, while the number of infected quarters decreased from 28·5 per cent. and 25·8 per cent. to 2 per cent. and 5·4 per cent. respectively. 7. The preventive measures institued resulted in the infection in first calf heifers being reduced from 60 per cent. in those that calved before the control scheme was put into operation to 20 per cent. in heifers calving subsequent to its introduction. It is probable that a large proportion of the latter animals were already infected prior to the commencement of the scheme. 8. Infection in some quarters was very obstinate and one case is recorded which required 18 infusions before it stopped secreting mastitis streptococci in the milk. 9. A reappearance of streptococci in the milk of quarters which appeared to have been cured is frequent and this necessitates one or more additional courses of treatment for such quarters. 10. Ninety-five out of 182 quarters which are claimed to have been cured have given negative results consistently for 10 months or more. The remaining 87 quarters have been clean for periods varying from 2 to 9 months. 11. Quarters with latent infection responded to treatment more readily than those with clinical mastitis, and do not show the same tendency to recurrence. 12. The figures available for the determination of the incidence of infection in the different lactation periods are unreliable since many of the older cows have become negative on account of auto-sterilisation or treatment by the owners. 13. For infusion therapy Entozon, Acriflavine, Rivanol and Euflavine were used in various concentrations, and the solutions were left in the udders for varying periods. There does not appear to be much difference between these drugs as regards efficiency though a slightly higher percentage of failures is recorded against Rivanol, while Euflavine promises to be more effective. 14. The periods for which solutions can be left in the quarters of cows in full lactation may be extended from 15 minutes to 30 or even 60 minutes without harmful results. 15. The number of quarters which are unintentionally rendered functionless by infusion is approximately 2 per cent. Some of these, however, resume their secretory activity in the following lactation. 16. The milk yield of the infected cows for the lactation during which they were being tested and treated for mastitis is compared with the expected yield for the same period, the latter being calculated from the yield in the previous lactation by making the necessary corrections for advancing age. Eighteen cows produced more and 16 less than their expected yields. On the whole the 34 cows gave 10,169 lb. (an average of 299 lb. per cow) more than their expected yield. 17. Fifteen of the 16 cows whose yield was not up to expectations had clinical mastitis, and it is probable that the degree of induration rather than the effect of treatment is responsible for the reduction in yield. 18. Butterfat percentage does not appear to be affected to an appreciable degree by treatment. 19. An example is given of the marked depression in yield caused by mastitis and of the ability of some udders with advanced clinical mastitis to recover a great portion of their secretory activity after treatment.