This investigation found that the human antibody class of importance in neutralizing tetanus toxin in mice was IgG, and that toxin neutralization was retained by the F(ab')2 and Fab' subunits of the human IgG class. Although human IgM and IgA classes appeared to neutralize tetanus toxin at very low levels, evidence was obtained that this neutralization was probably due to IgG contamination. Human Fabmu isolated from the IgM class did not neutralize tetanus toxin. Human antibodies of the IgG, IgM and IgA classes reacted with tetanus toxoid in the indirect haemagglutination (HA) test with IgG giving the highest HA titre. Rabbit antibodies of the IgG class also neutralized tetanus toxin, with neutralization being retained by the F(ab')2 and Fab' subunits of the rabbit IgG class. Absorption of several rabbit antisera to tetanus toxoid with goat-antirabbit Fc which is specific for absorption of IgG from antiserum, rendered them incapable of neutralizing tetanus toxin.