Since more or less severe nitrification inhibition was observed in several pilot and full-scale activated sludge plants, a means to quantify the extent of inhibition under operational conditions has been sought. Thus a method for the estimation of the actual maximum autotrophic growth rate μ^A,T was developed. This parameter helps to detect low nitrification capacity, before the effluent ammonia concentration increases. Two case studies show that nitrifying activated sludge plants may become acclimatized to inhibitory compounds but have to be protected from peak loads of both nitrogen and inhibitory compounds.