An experiment with 36 fattening friesian bulls was conducted to investigate the effect of flavomycin (50 mg/d.animal). The additional flavomycin significantly enhanced the apparent digestibility of the organic matter, crude fibre and NFE. The animals in the experimental group consumed significantly more feed than the controls until the 56th day of the experiment. No appreciable differences between the two groups occurred thereafter. The daily gain of the animals in the experimental group was always higher than that of the controls except from day 140 to day 168 of the experiment. The mean daily gain for the animals in the control group was 949 g, whereas for those in the control group it was 916 g. This difference is statistically significant. The daily gains of the experimental animals were 3.5% higher than those of the controls. The animals receiving flavomycin tended to need a lower energy input than the controls. The control animals had a mean requirement of 3.80 kEFr/kg gain compared with 3.68 kEFr/kg for the experimental animals. This corresponds to a 3.2% improvement in energy conversion. As far as slaughter performance was concerned, the additional flavomycin had no effect on carcase composition, organ weight and chemical composition. Only the internal fat fraction was increased somewhat by flavomycin. The flavomycin had no influence on the fatty acid composition in the rumen. In particular, the ratio of acetic acid to propionic acid was unchanged. A trend to a slight increase in the total fatty acid concentration was observed. The treatment did not affect the number of protozoa in the rumen.