This study investigated the inhibitory effects of Cd, Ag, Hg and Pb on Anammox biomass by batch experiments. Inhibition effects and recovery performance were monitored in sequence over 24 h and 72 h experimental runs. The IC50 (the 50% inhibitory concentration of nitrogen removal rate) values were calculated as follows: 11.16 +/- 0.42 mg/L for Cd; 11.52 +/- 0.49 mg/L for Ag; 60.35 +/- 2.47 mg/L for Hg. Pb caused only 7.19% decrease on nitrogen removal rate (NRR) even at a dose of 40 mg/L. Hence, the order of the inhibitory effect was determined as Cd > Ag > Hg > Pb. Furthermore, the crude enzyme (hydrazine dehydrogenase) activity and heme c concentration were also depressed under heavy metals exposure. Among four metals, Cd and Hg could exert sustained toxicity on Anammox biomass, but mal-effects of Ag and Pb could resume to a large extent after removing these two metals within 96 h. (C) 2014 Elsevier B.V. All rights reserved.