The induction of acid-shock proteins and the degree of acid resistance conferred on Clostridium perfringens by acid shock, and the kinetics of this resistance were determined. A sublethal acid shock at pH 4.5 for 20 min increased the acid tolerance of cells at least fifteenfold. The acquired tolerance was maintained for 3 h after acid treatment. The response of the microorganism to acid shock was also examined by analysis of pulse-labeled proteins. Five acid-shock proteins (molecular weights 120, 84, 58, 45 and 17 kDa) were identified by polyacrylamide gel electrophoresis.