A laboratory-scale anaerobic down-flow fixed-bed reactor, operating at 35 degrees C, was used to treat abattoir wastewaters at input soluble chemical oxygen demand (GOD) levels ranging from 4.7 to 28.7 g dm(-3) and volumetric organic loading rates ranging from 2.5 to 25 g COD dm(-3) day(-1). Start-up of the reactor was achieved within 35 days using a regime that included stepped increases in input COD and methanol substitution. The effects of the organic volumetric loading rate on soluble COD (SCOD) removal were studied at four feed SCOD concentrations between 4.7 and 28.7 g dm(-3). An exponential dependency of the organic volumetric loading rate with the SCOD removal was observed in the range of loadings studied, with variation between 1 and 12 g COD dm(-3) day(-1) Further increases of the organic loading rates had smaller effects on SCOD removal. A pseudo first-order kinetic model was used to obtain the gross removal coefficients, the values of which were virtually independent of the feed COD concentration at 6.9 to 7.3 g COD dm(-3) day(-1). The methane yield coefficient decreased slightly from 0.34 to 0.31 dm(3) g(-1) COD when the feed SCOD concentration was increased from 4.7 to 28.7 g dm(-3).