Michal Federowski's study entitled 'Lud bialoruski na Rusi Litewskiej' (The Belarussian People in Lithuanian Ruthenia) was the result of more than a dozen years of the field research he conducted in so-called Lithuanian Ruthenia. In 1889, Federowski had been commissioned by the Anthropological Committee of the Academy of Learning to draft a comprehensive ethnographic description of the region, and in 1897 he produced the first volume of 'Lud bialoruski...', in which he included, among other things, materials relating to traditional folk medicine. The current paper is an attempt to provide an analysis of the materials on folk medicine collected by Federowski, and in particular to assess the substances used in it, from the perspective of contemporary pharmaceutical knowledge, as well as to look for other, non-empirical and non-practical considerations underlying the use of some medicinal plants. A comparison of the plant descriptions by Federowski with contemporary knowledge and current uses of such plants in herbal medicine shows that the Belarussian folk medicine relied mainly on plants whose composition and curative properties have been well confirmed by contemporary research: mint, thorn apple, nightshade, poppy, and hemp. Apart from the above plants, Federowski's descriptions also included species whose use in traditional folk medicine differed from that recommended in contemporary herbal medicine (e.g. plants used for fumigations), or whose composition and action remain rather poorly investigated or totally unexplained. The number of such plants is much smaller, and they may provide material for further research. Especially noteworthy among such plants are those that played a key role in folk medicine and which were attributed with special properties, e.g. adder's tongue or quaking grass. These plants functioned in the traditional lore and folk culture not only as curative agents but also as objects of beliefs, legends, and songs, which endowed them with an additional spiritual dimension and frequently had a decisive role for their medicinal use.