As part of the assessment of product consistency and putative potency, it is often necessary to check not only that a vaccine contains the desired antigens, but also that they are in the correct immunological form (see Note 1). In general, this immunological characterization of a vaccine is interpreted as the ability of the preparation to invoke an appropriate immune response to its major constitutent antigens. The immune response in question may be in the form of serum antibodies, secretory antibodies, cell-mediated immunity (CMI), or a combination of these. The best approach to characterization of the immune response may be qualitative, such as immunoblotting to check for the presence of antibodies to specific antigens (1), quantitative, such as immunosorbent assays to measure the level of the antibody response to a particular antigen (2), or functional, such as toxin neutralization (3) to assess the magnitude of the functional activity of the response. The nature of the vaccine to be characterized and how its protective efficacy is mediated must be considered when deciding which aspects of the immune response should be examined and how they will be assessed.