Streptococcus is classified based on

Hemolytic streptococci were classified by Lancefield serologically into groups based on the nature of a carbohydrate (C) antigen on the cell wall. These are known as Lancefield groups, twenty of which have been identified so far and named A-V (without I and J). Reference: Textbook of Microbiology; Anathanarayan and paniker's; 10th edition; Page no: 211