The effects of dietary inclusion of yeast culture (YC) and of gradually increasing proportion of sugar beet molasses [0 (M₀), 100 (M₁₀₀) and 200 (M₂₀₀) g kg^-1] in the concentrate mixture on feed intake, live weight gain and carcass characteristics of growing Ayrshire bulls were examined in a 3 x 2 factorial design. Twenty-eight animals with a mean initial liveweight (LW) of 268 kg were used. Molasses replaced a part of concentrate, which was based on a mixture (1:1) of barley and sugar beet pulp. Grass silage was offered ad libitum and the concentrate supplements at the rate of 100 g per LW^0.6 on air dry basis. YC (5 g day^-1) was top-dressed on the concentrate. Feed intake was not affected by the treatments but LW gain decreased linearly (P < 0.05) with increasing rate of molasses (1.21, 1.16 and 1.09 kg day^-1; SEM 0.029). As the proportion of molasses increased from 0 to 200 g/kg concentrate the efficiency of feed conversion declined (linear effect P < 0.01) from 6.10 to 6.75 kg DM kg^-1 LW gain. YC had no significant effect either on feed intake (7.26 vs 7.49 kg day^-1) or LW gain (1.14 vs 1.17 kg day^-1). Yeast interacted with concentrate so that with M₀ LW gain was increased and with M₂₀₀ decreased by YC. The differences in carcass weight and carcass gain reflected a pattern similar to those in LW gain. Dressing proportion and fat grades were not affected by the treatments but carcass quality grade was better in animals receiving YC.