Protein L is a cell surface protein from Peptostreptoccocus magnus that binds to the variable light chains (kappa chain) of immunoglobulins without interfering with antigen binding. In contrast to IgG-binding proteins, such as protein A and protein G, protein L can be used for the detection and purification of mammalian kappa light chain antibodies of all classes. Biotin-Protein L, which is produced by covalently linking biotin to protein L, can be used in immunodetections, such as immunoblotting (western or dot blot), ELISA, and immunohistochemistry.