1. Heparin was more satisfactory for preventing blood from clotting than defibrination, potassium and ammonium oxalate, or sodium citrate in bacteriostasis of group A streptococci in the presence of streptococcal antibodies in convalescent serum.
2. Blood from rabbit, guinea pig, or sheep could not be substituted for human blood in promoting bacteriostasis when human antibody was used. Mixtures of human leukocytes and plasma of each of these animals or of animal leukocytes and human plasma were also not effective with human antibody.
3. Complement, leukocytes, and a thermostable factor which was found in human plasma were essential in the indirect bacteriostatic technique employed for the inhibition of streptococcal growth in the presence of convalescent human serum.
4. The thermostable component was active in human serum, as well as in plasma, in 1:12 dilution, withstood storage at 4° C. for at least 7 weeks, and was destroyed by heating at 70° C. for 30 minutes.