The influence of salinomycin (0, 15, 30 and 60 mg per animal and day) on apparent digestibility of artificially dried grass was investigated in four experiments with five wether each. Three long term individual feeding experiments (210 to 252 days) with 72 growing bulls were carried out. The influence of various salinomycin levels (100 to 300 mg per animal and day) were investigated on parameters of rumen fermentation as well as fattening and slaughtering results. Salinomycin decreased insignificantly (P greater than 0.05) the apparent digestibility of organic matter (71.0; 70.1; 68.7 and 68.4%) and crude carbohydrates (71.8; 70.3; 69.2 and 68.5% resp.). The digestibility of other nutrients was not influenced. Supplementation of salinomycin reduced molar concentration of acetate (49 to 115) and butyrate (22 to 82) and increased propionate (110 to 199 mmoles per mol) in rumen liquid. Dry matter intake of bulls declined (0.8%, 13.5 and 24.6; 4.7% of experiments 1 to 3) when salinomycin was added. Daily weight gain (6.5 and 1.0%) and feed efficiency (6.8 and 6.3%) of bulls supplemented with salinomycin were improved in experiments 1 and 3. Significant decrease of feed intake of bulls of experiment 2 affected decline of weight gain probably as consequence of high salinomycin levels. Slaughtering results (except experiment 2) and body composition of bulls were not significantly influenced by salinomycin. Under consideration of own results and some references 10 to 20 mg per kg dry matter or 50 to 150 mg salinomycin per fattening bull and day are recommended as optimal level.