The effect of supplementing a diet of wheat straw plus urea with either 0, 100 or 200 g/d cottonseed oilcake meal .(CSM) in combination with either 0, 100 or 200 g/d whole maize on wool and carcass growth of SA Mutton Merino lambs was studied. Voluntary intake of wheat straw was increased by CSM and decreased by maize (P < 0,01). Feed conversion ratio was improved by both supplements (P < 0,01). Although growth rate was increased by both supplements (P < 0,01), the response to CSM supplementation was almost double that for maize. The proportion of fat in the carcass was doubled by maize (P < 0,01), but was not affected by CSM. The proportion of fat in viscera was increased by both supplements (P < 0,01). Wool growth rate was increased by 89% and 226% when 100 or 200 g/d CSM were fed respectively, and by 46% and 96% when corresponding amounts of maize were fed. Wool fibre diameter was increased by 26% with 200 g/d CSM and by 6% with 200 g/d maize. Acetate clearance rate was not significantly increased by maize or CSM (P > 0,05). An interaction between treatment effects for feed conversion ratio suggests that responses to protein supplementation will depend on the level of energy supplied.