The hydrolysis of soluble proteins in an anaerobic, saline (24g dm(-3) NaCl) and mesophilic (37degreesC) environment was studied. The inhibitory effect of a volatile fatty acid, acetic acid (HAc), on the hydrolysis rate and hydrolytic biomass activity for a model saline wastewater with a high protein load (total organic carbon, 1153mg dm(-3) and 1572mg dm(-3) proteins) was studied. Initial inhibitor concentrations were tested in the range of 0-2000mg dm(-3) HAc. The microbiological characterization was performed using a total microorganism count by epifluorescence, and hydrolytic bacterial activity was determined by plate count. The protein hydrolysis was modeled according to first order kinetics. The effect of biomass on hydrolysis was analyzed by varying its concentration in the range of 42-210mg dm(-3) volatile suspended solids. The following apparent hydrolysis kinetic constants (K-h) for proteins at 37degreesC were obtained: 1.3, 0.8, 0.6, 0.2 and 0.1 d(-1) for initial concentrations of 250, 500, 750, 880, and 1000mg dm(-3) HAc, respectively. At concentrations of HAc greater than 1000mg dm(-3), total inhibition of hydrolysis was observed. The intrinsic hydrolysis constant (K-h(0)) at 37degreesC, without inhibition, was 2.3 d(-1). The hydrolysis kinetic constant was not affected by the biomass concentration. The hydrolysis kinetics constant was filted to three models: Luong, Levenspiel and non-competitive inhibition. The model that best represented the experimental data was Luong, obtaining an inhibition constant (K-I) of 1087mg dm(-3) of HAc and the exponent gamma = 0.54. The hydrolysis was inhibited by the presence of HAc, which corresponds to an intermediate compound of the anaerobic process. (C) 2004 Society of Chemical Industry.