Fifteen men, 30–40 years old, who had received one dose of vaccine against tetanus, were given a booster dose 8–13 years later. Their serum content of tetanus antitoxin was determined (1) before the booster dose, (2) 3–4 days and (3) 8 days after the booster dose. A control group of ten men with 2–3 earlier vaccine injections was treated in the same way. All the men in the first group had an antitoxin content of 0.01 IU/ml or more, 8 days after the booster dose, while the comparative titres increased to more than 1 IU/ml in the control group. The advantage of a complete basic immunization is obvious, but the investigation shows that, even after one injection, the titre after a booster dose increased to a generally accepted protective level.