It was found earlier that fresh tetanus toxin and tetanus toxoid carry two antigenic determinants each of which elicits tetanus antitoxin in rabbits and man. In this paper the purification of a degraded tetanus toxin carrying only one antigenic determinant is described. It was shown in quantitative precipitation studies that the antitoxin corresponding to this determinant could be absorbed from antitoxic rabbit sera and human immunoglobulin confirming earlier findings. Both antibodies could be isolated from antigen-antibody precipitates. Their reactivity in double diffusion tests with fresh tetanus toxin and degraded toxin was analyzed. The degraded toxin was shown to be antigenic in rabbits and to elicit specific antitoxins.