This monograph deals with bactericidal substances in serum that differ from the bacteriolysins mainly in being more resistant to heat. The author, who has done a great deal of work on these substances, calls them serum-β-lysins to distinguish them from the less heat resistant lysins, which he calls serum-α-lysins. The properties of the serum-β-lysins, the bacteria that are susceptible to their action, and the immunity against some of these bacteria are discussed. The monograph will interest all those who are working with the bactericidal actions of the blood and its components. The literature in question is reviewed thoroughly.