It has been shown that modified fluid gelation (MFG) was antigenic in the rabbit only upon injection in a water-in-oil emulsion. MFG was not antigenic in man upon 5 intramuscular injections. Further studies with MFG are indicated before definite conclusions are reached as to its antigenicity in man. It appears that a great part of the serological specificity of the antibodies to MFG is directed against the succinyl groupings. The effect of various acids on the solubility of the MFG-anti-MFG precipitates has been presented.