Monospecific equine antiserum against cholera exo-enterotoxin.

An antiserum specific for Vibrio cholerae exo-enterotoxin was produced by immunization of a horse with purified choleragenoid, a natural cholera toxoid. The serum has a high titer against the toxin antigen in passive hemagglutination tests and a respectable antipermeability factor activity. It also passively protected against choleragen-induced mouse foot edema. The serum was found to be useful for assaying toxin antigen in crude and refined products by in vitro tests such as radial immunodiffusion, Lf, and quantitative precipitin titrations. Based upon experimental observations, the serum was defined as containing 1,000 flocculating units of anticholera toxin antibody per ml. A flocculating dose, or Lf, of choleragen approximates 1 mug, and that of choleragenoid is 0.625 mug. Formalin toxoids behaved like the parent toxin in these tests. The serum contains approximately 2.2 mg of antibody protein per ml, which appears to be largely, if not entirely, of the Ig(T) type. It is suggested that this serum, which is available in considerable supply, be considered for use as a reference cholera antitoxin. The horse developed symptoms of anaphylactic shock during immunization, suggesting the need for caution in projected studies on toxoid-induced immunity in man.