Serological classification of Cl. botulinum, based on the antigenic structure of the toxins produced, is distinguished by the behaviour of the toxin-antitoxin mixtures in in vivo neutralization tests. Observations on the dissimilarity between strains within types, the behaviour of antitoxins in the cross-neutralization tests, the established concepts of the antigenic structure of the toxin types and the lack of a standard methodology for typing, have led to the definition of the terms efficiency, type, subtype, intratypic serological variant (ISV), and degree of serological homology. These definitions are primarily applied to typing and to the establishment of the taxonomic serological categories of type, subtype and ISV. In the case of antitoxin standardization, the accepted standard methods must be followed.