The Authors carried out a research work to study the dose-dependence of antibody response in BD2F, mice injected with tetanus toxoid. Experimental data show that it is possible to obtain an earlier resistance at least 14 days after immunization, increasing ten-fold the usual anatoxin dose administered in humans. The protection test toward 4 MLD i.v. injected of tetanus toxin proved a valuable tool for assessing the first appearance of acquired immunity.