Studies were conducted to evaluate the capacities of an activated sludge in degradation process for a xenobiotic. The results showed that during its acclimation to 2,4-D, the sludge sharply accumulated degradation capacities before degradation could proceed at a noticeable rate; moreover, during de-acclimation the sludge gradually lost some capacities in a manner resembling an exponential decay. (c) 2006 Elsevier Ltd. All rights reserved.