It was shown that the immunity induced by a primary and secondary dose of antigen varying from 25 to 135 Lf APT, protected sheep for only about nine weeks. Any booster dose given before a year after the primary injection resulted in protective titres lasting for periods up to about 20 weeks. When a sound basic immunity is established, e.g. with two doses of 135 Lf, sheep are protected to the same extent by a large (135 Lf) or small (25 Lf) booster dose given one year after the primary injection. The protection thus effected lasts for at least one year. When sheep with a sound basic immunity receive a booster of 135 Lf after one year and a second booster of 135 Lf after a further year, they are protected against pulpy kidney disease for at least three years following the last injection. A single injection of epsilon toxoid in Freund's complete or incomplete adjuvant protects sheep for at least two years.