Heavy metals could potentially have a negative impact on methane-producing anaerobic granular sludge. The objective of this study was to investigate the inhibitory effect of zinc(II), chromium(VI), nickel(II), and cadmium(II) on the methane-producing activity of granular sludge sampled from the upflow anaerobic sludge blanket reactor that treats the wastewaters of a yeast factory, for a range of concentrations between 0 and 128 mg L-1. The modified Gompertz, Logistic, and Richards equations were applied to describe the inactivation of anaerobic culture by heavy metals. According to these models, the values of methane production potential (mL) for a heavy metal concentration of 128 mg L-1 were in the following order: Ni (44.82 +/- 0.67) > Cd (28.73 +/- 0.11) > Cr (15.52 +/- 1.63) > Zn (0.65 +/- 0.00). The IC50 values, the metal concentrations that cause a 50% reduction in the cumulative methane production over a fixed period of exposure time (24 h), for the individual heavy metals were found to be in the following order: Zn (most toxic; 7.5 mg L-1) > Cr (27 mg L-1) > Ni (35 mg L-1) approximate to Cd (least toxic; 36 mg L-1). (C) 2008 Elsevier B.V. All rights reserved.