At high toxoid concentrations as well as at low pH values the curves of the salting out process of diphtheria toxoid with ammonium sulphate have the classical S shape. At lower toxoid concentrations and at a pH value a little below 7 or above it, a part of the curve is horizontal or in some cases even descends. This suggests the presence of two flocculating components, one of which (component A) is precipitated at low (NH4)2SO4 concentrations; this component is probably not immunogenic, or only poorly so. Fractionation and gel diffusion experiments confirmed the presence of these two components. In certain fractions of the toxoids the components may manifest themselves separately by the presence of two flocculation zones.