Summary 1. The blood and milk levels of unconjugated sulfanilamide in cattle were maintained over a period of 12 hours either following an initial dose or after the last dose of a period of treatment. This insures a reasonably constant level of sulfanilamide in the blood of cows that are dosed twice daily at 12-hour intervals. 2. It was possible to attain a level of sulfanilamide in blood and milk slightly under 8mg. per 100 cc. only when the dose was approximately 10 grams per 100 pounds body weight or twice that recommended for man. Blood levels slightly less than 2mg. per 100 cc. were attained with a dose comparable with that recommended for man, that is, 5 grams per 100 pounds body weight daily. Both of these levels are below that of 10mg. per 100 cc. of blood suggested for favorable results in man. 3. Doses of 5, 10, or even 15gm. per 100 pounds body weight over a period of 3 to 10 days failed to permanently eliminate Beta Hemolytic Streptococci from the udders of cows affected with streptococcic mastitis, regardless of whether the cases were acute or chronic or of short or long duration. Even recently affected non-clinical cases were not freed of the organism. 4. Symptoms of acute streptococcic mastitis such as tenderness, swelling, hardness of the quarter, accompanied by flaky, pussy, or watery milk were relieved in most cases by administering sulfanilamide in doses of 5 to 10 grains per 100 pounds body weight for 7 to 10 days. Five grams per 100 pounds body weight seemed to be as effective as larger doses in relieving clinical symptoms of acute mastitis. The cases treated had failed to respond satisfactorily to the standard treament of applying hot packs, frequent milkings, massages, laxatives and udder ointments. 5. Sulfanilamide poisoning in the form of sluggishness, loss of appetite, reduced milk flow, roughened coat, fever and increased pulse and respiration were produced in 1 to 3 days when the total daily dose was 15 grams per 100 pounds body weight. Five grams per 100 pounds body weight had little or no detrimental effect in 9 of 11 cows treated. The dose should be reduced or eliminated when toxic symptoms appear. 6. One cow died following doses of 10 and 15 grams per 100 pounds body weight with enteritis diarrhea and definite cyanosis of the musculature. Another cow showed extensive eczema when allowed contact with the direct rays of the sun during treatment with 10, and later 5, grams per 100 pounds body weight. There seems to be an individual difference in tolerance of cows to sulfanilamide. 7. Sixteen cows were treated for streptococcic mastitis with sulfanilamide. Eight of nine cases with an initial infection showing acute symptoms gave favorable results with a reduction of inflammation of the udder and restoration of normal-appearing milk. One cow showed symptoms of toxic poisoning and treatment was discontinued. Favorable results were obtained in 6 out of 9 severely affected quarters in 4 old chronic cases. The 3 additional quarters were greatly improved. No improvement was shown in three cases of initial infection where clinical symptoms had not developed.