Four groups each of eight Limousin X Friesian bulls, average weight 344 kg, were offered individually ad libitum diets comprising: 920 g rolled barley, 50 g cane molasses, 15 g urea and 15 g minerals/vitamins mix per kg or 867·5 g rolled barley, 50 g cane molasses, 7·5 g urea, 60 g soya-bean meal and 15 g minerals/vitamins mix per kg, plus or minus yeast culture (YC, 1·5 kg/t) (all weights fresh weights), until slaughter (460kg). Rumen samples were obtained by stomach tube. YC decreased rumen ammonia concentrations (116·7, 56·0, 74·3, 45·7 (s.e.d. 19·40) mg/l for barley/urea, barley/urea/YC, barley/soya-bean meal and barley/soya-bean meal/YC, respectively). This was associated with increases (P <0·05) in both the total number of bacteria (2·3, 4·2, 3·5, 6·8 (s.e.d. 1-06) X 10 per ml, respectively) and the protein content of rumen fluid (7·7, 9·6, 8·7, 9·0 (s.e.d. 0·59) g/l, respectively). However, the flow of microbial protein from the rumen as estimated from the appearance of purine derivatives in the urine was unaffected (131, 130, 129, 147 (s.e.d. 12·6) g N per day, respectively). Thus although the animals responded to the undegradable rumen protein supplied by the soya-bean meal there was no significant response in live-weight gain with YC (1·28, 1·31, 1·42, 1·48 (s.e.d. 0·081) kg/day, respectively).