Tests among 410 Indians not artificially immunised against tetanus showed that 80% had measurable antitoxin. Single doses (100 Lf or 250 Lf) of a potent tetanus toxoid were given to such individuals with naturally acquired antitoxin. The 100 Lf dose produced on average a ten-fold rise in antibody level, and the 250 Lf dose a twenty-fold rise. In adults who had been artificially immunised, a 5 Lf dose produced a four-fold to ten-fold rise in antibody level. In infants three doses of triple vaccine produced satisfactory antitoxin concentrations. The levels of antibody achieved after a single 250 Lf dose should protect for 5 years. Single-dose vaccination may be better than the conventional three-dose scheme for a population that is unlikely to comply with a three-dose regimen and in whom naturally acquired antitoxin is associated with partial tolerance to tetanus toxoid. Naturally acquired antitoxin in Indians is probably the result of chronic clostridial contamination of the small bowel. This contamination can induce immune tolerance in the gut and systemically and may be the reason for the poor responses to vaccination in all except infants.