The effect of lasalocid and cationomycin on plant cell wall digestion was tested in a Latin square design experiment over three periods on six adult sheep fed three diets: a control diet (T) without antibiotics, a diet (L) with 33 mg kg−1 of lasalocid, and a diet (C) with 33 mg kg−1 of cationomycin. The dry matter and plant cell wall digestibilities were not affected by the addition of antibiotics. The digestive flow measurements at the duodenum showed that the antibiotic had no effect on the apparent digestion of dry matter, organic matter and plant cell walls along the digestive tract. This was confirmed by the in sacco feed and pure cellulose rumen degradation measurements. Lasalocid, however, decreased the true digestion of feed dry matter in the rumen, as shown by the duodenal flow measurements after being corrected for microbial dry matter. Compared with the control diet, diets (L) and (C) increased the propionate percentage in the rumen VFA mixture (T = 14.9, L = 19.4, C = 18.9) and decreased acetate (T = 66.1, L = 63.8, C = 65.7) and butyrate (T = 14.1, L = 12.7, C = 11.7) percentages. The addition of antibiotics decreased the rumen ammonia nitrogen concentration by 14%. The CO2 to CH4 ratio in the gas mixture was, however, not statistically modified, and no ionophore effect was observed on the protozoa mean population. Key words: Lasalocid, cationomycin, digestion, cell wall carbohydrates, sheep, rumen