The effect of an enzymatic pretreatment, Pancreatic Lipase 250 (PL-250), on the hydrolysis and size reduction of fat particles in slaughterhouse wastewater was characterised for enzyme doses ranging from 125 to 1000mg dm(-3) and initial particle sizes (D-in) varying between 53 and 383 mum. Treatment with PL-250 significantly reduced the size of pork fat particles in slaughterhouse wastewater. Particle size reduction increased with D-in, possibly due to the more filamentous and plate-like configuration of the larger fat particles, which could be easily broken at weak points. The smaller particles were observed to be denser and more spherical. Size reduction also increased with enzyme concentration, but the benefit of adding more enzyme diminished greatly as enzyme dose was increased. The maximum long-chain fatty acid (LCFA) concentration in filtered samples was detected after 4-7h of treatment and ranged from 8.2 to 34.9mg dm-3. The linear rate of LCFA released in solution during enzymatic pretreatment ranged from 39.4 to 169.9mg dm(-3) d(-1), and increased with enzyme concentration up to 500mgdm(-3). At a PL-250 concentration of 1000mgdm(-3), the LCFA release rate decreased, maybe due to excessive layering of adsorbed enzyme on the fat particles or increased degradation of released LCFAs. The pretreatment appeared to be more efficient with beef than pork fat particles. However, the effect of an enzymatic pretreatment on a downstream anaerobic treatment of slaughterhouse wastewater containing fat particles remains to be tested.