Twenty-eight Suffolk-cross male castrated lambs aged 7 to 8 months, initial live weight 36 (s.e. 149) kg were given four diets which contained unground pelleted molassed sugar-beet pulp and rolled barley (940 g/kg, fresh weight basis) in the following proportions either A, 0·8 to 0·2 or B, 0·5 to 0·5, with 60 g soya-bean meal per kg. Diets A and B were offered with or without a yeast culture product Yea-Sacc. Diets zvere offered twice daily to appetite with 100 g hay per head. Yeast culture had no effect on any measurement made (P > 0·05). Lambs given diet B consumed more dry matter (DM), grew faster and had superior food conversion efficiencies (all P < 0·01). Rumen fluid taken 3 h after feeding contained higher concentrations of ammonia (F < 0·01), total volatile fatty acids (F < 0·001) and acetic acid (F < 0·001) in samples from lambs given diet A. Rumen fluid samples collected before the 10·00 h meal, produced more gas from diet A than diet B after 6 and 24 h in vitro incubation (F < 0·001) suggesting the presence of more undigested food. When ground diets were incubated, more gas was produced from diet A after 6h (F < 0·05) but not after 24 h fP > 0·05). It is suggested that diet B supported faster growth of the lambs as a result of faster rate of digestion, higher DM intake and superior food conversion.