The article by A. A. Miles and S. S. Misra, with a note by J. O. Irwin, discusses the estimation of the bactericidal power of blood using Staphylococcus aureus as a model organism. The authors describe methods for measuring the number of bacteria inoculated and the bactericidal power of blood, emphasizing the importance of accurate colony counts and the use of Fildes' agar for surface-viable counts. They also address the impact of bacterial aggregation on colony counts and the significance of the survival rate \( p \) in reflecting the bactericidal power of blood. The study finds that healthy human blood has a survival rate \( p \) that differs significantly from that of patients with chronic staphylococcal infections, suggesting that individual variations in blood bactericidal power can be detected using their method. The note by Irwin provides a detailed statistical derivation of the standard error of the survival rate \( p \) in blood-bacterium mixtures.The article by A. A. Miles and S. S. Misra, with a note by J. O. Irwin, discusses the estimation of the bactericidal power of blood using Staphylococcus aureus as a model organism. The authors describe methods for measuring the number of bacteria inoculated and the bactericidal power of blood, emphasizing the importance of accurate colony counts and the use of Fildes' agar for surface-viable counts. They also address the impact of bacterial aggregation on colony counts and the significance of the survival rate \( p \) in reflecting the bactericidal power of blood. The study finds that healthy human blood has a survival rate \( p \) that differs significantly from that of patients with chronic staphylococcal infections, suggesting that individual variations in blood bactericidal power can be detected using their method. The note by Irwin provides a detailed statistical derivation of the standard error of the survival rate \( p \) in blood-bacterium mixtures.