Protective Action of Tetanus Toxoid Unrelated to Active Immunization in Mice.

1. Mice injected with a single dose (0.4 ml) of fluid tetanus toxoid 24 hours prior to, or simultaneously with a small dose (1 LD50 to 1 MLD) of tetanus toxin exhibited a degree of protection against the action of the toxin. The protective effect varied directly with the dose of toxoid. Toxoid given 3 or 5 days before the toxin afforded little or no protection. 2. Small daily doses of toxoid (0.1 ml) given over a 4-day period commencing 3 days prior, one day prior, or on the day of toxin administration were as efficacious in inhibiting the action of toxin as an equivalent single large dose of toxoid (0.4 ml) administered coincident with the injection of toxin. 3. When toxoid and toxin were mixed in vitro prior to injection into mice, the protective effect of the toxoid appeared to be enhanced.