Anti-digitoxin antibodies have been produced by immunizing rabbits and by monoclonal techniques using the same digitoxin-protein conjugate. Titres, inhibition curves, affinities, dissociation and association rate-constants have been investigated for the two antibodies. It appears that the animal immune serum gives better criteria (except specificity) than the monoclonal ones. However, both can be used for quantitative assays, with less sensitivity for the monoclonal antibody. The main advantage of monoclonal production is to conduct large amounts of specific immunoglobulins which are very useful for human administration after a digitalis overdose.