AbstractThe striking similarity of phenomena found in specific precipitation with those encountered in complex colloid flocculation (in the sense of Bungenberg de Jong et al.) affords reason to expect a similarity of reaction mechanism. By studying the influence of neutral salts on the flocculation of diphtheria toxin with antitoxic horse serum we have tried to answer the question whether electrostatic attraction is as essential for specific precipitation as it is for complex colloid flocculation.