Introduction. The need for standardizing methods of measuring the biological activity of toxoids is generally recognized. Antigenicity is frequently assayed by qualitative or crudely quantitative methods. Mueller et al (1), in reporting their work on tetanus toxoid, have emphasized the need for precise methods of determining antigenicity since they found that there were “tremendous individual variations among guinea pigs in ability to respond to identical stimuli”. Two methods have generally been used for the in vivo evaluation of toxoids. In one method immunity is challenged by injection of toxin, and the degree of resistance to this challenge is used as a measure of the antigenicity of the toxoid. In the other method the average serum antitoxin titer of the immunized animals is used as an index of the antigenicity of the toxoid.