This paper by Rebecca C. Lancefield, published in 1932, presents a serological method for differentiating hemolytic streptococci from various sources, including human, other animals, and dairy products. The study involved 106 strains of *Streptococcus haemolyticus* and aimed to establish a classification system based on a precipitin reaction. The classification is based on the presence of a group-specific carbohydrate, which is not type-specific but is characteristic of each group. The paper discusses the limitations of previous biochemical and cultural methods and highlights the advantages of the precipitin test in overcoming these limitations. The study found that 104 of the 106 strains could be classified into five groups: Group A (human origin), Group B (bovine and dairy sources), Group C (other animal sources), Group D (cheese), and Group E (certified milk). The classification is supported by both serological and cultural/chemical characteristics, with the serological method showing high specificity and consistency. The group-specific substances in Group A were identified as carbohydrates, while those in other groups remain unknown but are likely also carbohydrates. The paper also discusses the cross-reactions between groups and the susceptibility of certain strains to bacteriophage, which is correlated with the grouping. The results confirm the validity of the serological classification and provide a valuable tool for identifying the probable origin of specific strains of *Streptococcus haemolyticus*.This paper by Rebecca C. Lancefield, published in 1932, presents a serological method for differentiating hemolytic streptococci from various sources, including human, other animals, and dairy products. The study involved 106 strains of *Streptococcus haemolyticus* and aimed to establish a classification system based on a precipitin reaction. The classification is based on the presence of a group-specific carbohydrate, which is not type-specific but is characteristic of each group. The paper discusses the limitations of previous biochemical and cultural methods and highlights the advantages of the precipitin test in overcoming these limitations. The study found that 104 of the 106 strains could be classified into five groups: Group A (human origin), Group B (bovine and dairy sources), Group C (other animal sources), Group D (cheese), and Group E (certified milk). The classification is supported by both serological and cultural/chemical characteristics, with the serological method showing high specificity and consistency. The group-specific substances in Group A were identified as carbohydrates, while those in other groups remain unknown but are likely also carbohydrates. The paper also discusses the cross-reactions between groups and the susceptibility of certain strains to bacteriophage, which is correlated with the grouping. The results confirm the validity of the serological classification and provide a valuable tool for identifying the probable origin of specific strains of *Streptococcus haemolyticus*.