Dried leaves of Salvia officinalis L., a rich source of phytofactors, were evaluated for their effect on methane production, microbial population, and basic parameters of rumen fermentation in a batch culture system. A 400 mg mixture of meadow hay and barley meal at a ratio of 60:40 was used as the substrate. Dried leaves of Salvia officinalis L. were added to the substrate at six levels (4, 10, 20, 40, 100, and 200 mg). A wide range of dried Salvia officinalis L. leaves, in quantities even up to 50% of the substrate, was used to test the effect of increasing amounts of phytofactors in the investigated cultures, mostly on methane production. CH₄ production was significantly mitigated by supplementation with 200 mg of sage per incubation vessel. The supplement affected the protozoa and methanogen populations which indicates that methane emission is associated with these populations; in vitro dry matter digestibility decreased linearly, however. Although Salvia officinalis L. seems to be a plant for which there may be a use in ruminant nutrition and the presented results suggest that the optimal level of Salvia officinalis is between 100 and 200 mg, further investigations under long-term in vivo conditions are necessary, as are the determination of the optimum dose and the active agent.