Carbon δ¹³C and nitrogen δ¹⁵N isotopic analyses of archaeobotanical plant remains can provide essential information on medieval agriculture and medieval life. To understand local growing conditions and the cultivation regime used by medieval farmers, charred cereal grains from medieval contexts were selected for stable carbon and nitrogen analyses. Single grain analysis was used for barley and rye grains from Mankby in Espoo, Finland, and from Uppsala Street and Lutsu Street in Tartu, Estonia. The results showed that the δ¹⁵N-nitrogen values in Mankby were significantly high, suggesting manuring with fish remains and livestock dung. The grains from Tartu were probably fertilised with livestock manure. The possibility has also been raised that various substances such as household waste, fish waste, ash, animal dung and seaweed could have been used as mixed manure.