Thirty two Döhne Merino lambs with an initial mean (± s.d.) body weight of 30.3 ± 2.5 kg were used in a randomised block experiment to determine the effect of an exogenous fibrolytic enzyme preparation (Abo 374) on body weight gain, dry matter intake and feed conversion ratio. Wheat straw was pre-treated with different levels of the enzyme supernatant and mixed with lucerne hay and a concentrate mix just before feeding. Treatments were 10 (High), 5 (Med), 1 (Low) or 0 (Control) mL supernatant/kg wheat straw. For each treatment, the Abo 374 supernatant was appropriately diluted with water to allow an application rate of 300 mL of the solution/kg straw. In the Control treatment, the solution consisted of water alone. Following enzyme application, the wheat straw was stored uncovered in plastic containers at room temperature for a pre-feeding incubation time of 18 hours. Enzyme treatment significantly improved body weight gain and feed conversion ratio from 0-6 weeks. Cumulative weight gains (kg) and feed conversion ratios (kg gain/kg dry matter intake) were 6.8 and 0.15, 7.1 and 0.16, 5.5 and 0.13, and 5.4 and 0.12. Treatment had no effect on dry matter intake. It was concluded that fibrolytic enzymes produced by Abo 374 are able to increase cell wall degradation and have potential to be used in the animal feed industry.